As a former e4 - e5 player, I'll be blunt:  I skimmed through this book for about 20 minutes in a book store a couple of days ago.  It cost $23.95 American money.  It's worth maybe $2.00.  The book is obviously what they used to call a "potboiler" back in the 1930s and 1940s...something to pay the rent, put food on the table, and pay the gas bill to keep the pot boiling.

Both Emms and Davies treated some openings rather casual. Their analysis didnt go deep enough or
they neglected some important sources. Emms book is quite good, still containing a lot of original analysis. Davies book is reasonable, but sometimes just copying Emms or taking a shortcut recommending a variation that is not quite sound. 

Marin tells us in his interview that he spends a lot of time on this project. And given his excellent writing style I have high hopes this will prove to be a book that will contain a solid repertoire and quite a few original ideas.

It is also worth noting that Kaufman's black repertoire is based around 1.e4 e5.  Still, I like having several different recommendations to choose from.  I have Emms, Kaufman, Davies and find them all useful in different ways.

I also have the Watson/Schiller How to Survive and Beat Annoying Chess Openings which has some good material too (the stuff written by Watson mostly!).

Sounds like overkill. First we had Emms, then Davies followed, and now Marin. Except in the Ruy Lopez the developments in the Open Games are not that fast, that we need an update about every year.

I've had this book for a few weeks now, and have spent some time going through it.  I have a few comments to add to the ones made already:

In general, I like Davies' decision to propose a Closed Variation of the Ruy Lopez, partially because of the flexibility playing the Closed Variations gives the player of the Black pieces.  Just in the Chigorin Defense alone, there are a half dozen different systems for Black that are completely playable and are suitable to players of a variety of temperaments and styles.  And on the 9th move, Black can chose among the Breyer System, the Zaitsev Variation, the Karpov Variation, the Smyslov Variation, etc.  Once you learn the theory up to the main Ruy Lopez tabiya, which occurs after 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.0-0 Be7 6.Re1 b5 7.Bb3 d6 8.c3 0-0 9.h3, you really have a wealth of sound and sharp options as Black.

I also like Davies' choice of Graf Variation of the Chigorin Defence, because this is a very tense and interesting variation in which both players can play for a win.  However, it would have been nice if Davies could have presented a more positional variation of the Chigorin as well, because in several places in his analysis he notes that Black's best line is probably to take a perpetual (see game 1, notes to move 19 and 21), thus making the Graf Variation a poor choice in a must-win situation (at least against someone who knows a lot of theory and wants a draw with the White pieces).  The Graf variation does appear to be a good line to play against stronger players, as it seems that White must take real risks to try and play for a win in some of the main lines.  To be fair, space constraints imposed on Davies may have prevented him from covering another main line option.

The above comment is a minor quibble, hoever, because by getting you to the move 9 tabiya Davies has given you a lot of options.  For those looking for a back-up line to play when a draw is not desirable, a few of the many playable options are (i) for the positional player, the deep main line Chigorin which occurs after 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.0-0 Be7 6.Re1 b5 7.Bb3 d6 8.c3 0-0 9.h3 Na5 10.Bc2 c5 11.d4 Qc7 12.Nbd2 cxd4 13.cxd4 Nc6 14.Nb3 a5 15.Be3 a4 16.Nbd2 Bd7 is extremely solid for Black and presents possibilities for playing for a win in a positional maneuvering game (there are some White deviations on the way that you need to know, of course, 14.d5 being perhaps the most important); (ii) for the more aggressive player, perhaps 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.0-0 Be7 6.Re1 b5 7.Bb3 d6 8.c3 0-0 9.h3 Na5 10.Bc2 c5 11.d4 Qc7 12.Nbd2 cxd4 13.cxd4 Bb7 14.d5 Rac8 15.Bd3 Nd7 16. Nf1 and now either 16...f5 immediately or 16...Nc5 with the idea of 17...f5 next, both of which are covered in an excellent article by Mihail Marin in NIC Yearbook 74 and seem to lead to complex play; or (iii) Romanishin's Gambit, which occurs after 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.0-0 Be7 6.Re1 b5 7.Bb3 d6 8.c3 0-0 9.h3 Na5 10.Bc2 c5 11.d4 Nd7 12.Nbd2 cxd4 (instead of 12...exd4 as in the Graf Variation) 13.cxd4 Nc6 14.Nb3 a5 15.Bd3 a4!? 16.Bxb5 Qb6 17. Bxc6 Qxc6 18.Nbd2 Bf6, which gives Black good play for the pawn according to Alexander Galkin's article in NIC Yearbook 76.  

For those interested in exploring other options, I think Glen Flear's "The Ruy Lopez Main Line" (Everyman 2004) is an excellent supplement to Davies' book because the first 150 pages or so of this book deal with Black's various options after 9.h3.  I would also recommend Flear's book, which is also in Everyman's "complete games" format,  because it includes a good summary of the theory in all of the major Closed Ruy Lopez variations.  (And you never know.  If you are one of those guys who only drinks decaffeinated coffee, gets regular checkups and always carries an umbrella "just in case," then the Breyer may be the opening for you!)

In general, I like Davies proposed lines against earlier deviations by White from the Ruy Lopez mainline.  A few comments: (i) the piece sac in the DERLD line mentioned in TalJechin's post actually seems quite promising for Black, who has scored a whopping 71% in the 28 games I found on ChessBase's online database (it appears that no GM playing the White pieces has allowed this line since Conquest lost with White to Stefansson in 1992); (ii) I don't see any coverage on the 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.d4 line, which often transposes to the 6.d4 lines but has some independent variations where White delays castling, and because this line is popular at the amateur level it probably should have been covered briefly (a few minutes with a database will suffice to cure this omission, as this line is not promising for White); and (iii) I like the choice of 5...Qf6 in the Exchange Lopez after 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Bxc6 dxc6 5.0-0), which appears very interesting ("deserves serious attention..." -The Ruy Lopez Exchange, Panczyk and Ilic [Everyman 2005]), and although in the past I have had good experiences in the main line with 5...f6 and with 5...Bd6 (which, sadly, no longer appears to be entirely playable), Davies' suggestion appears to be a useful and less theoretical alternative.

Having always played the Two Knights Defense myself, I was happy to see that Davies chose it for his repertoire, and I think the coverage is quite good.  The 4...Bc5 lines are definitely the most practical choice against the Scotch (you need a lot of theoretical knowledge to play 4...Nf6 these days), and I think Davies' chapter on the Scotch is really excellent, one of the best in the book.  I haven't spent any time looking at Davies' recommendation against the 4 Knights or the Goring Gambit, but Davies seems to have done a good job of chosing practical lines against the Vienna and King's Gambit.

Overall, my impression of this book is highly favorable.  Davies does a excellent job providing you with a playable Black repertoire after 1.e4 e5 in a limited amount of space and a limited number of carefully-chosen games.  Although there are bound to be some analytical errors and some omissions in a book of this scope, the general quality of line selection and analysis appears to be quite high.  And one of the joys of playing a really sound opening like 1.e4 e5 is that, if you don't like a particular line suggested by Davies, you will always have options.

I would strongly recommend a Closed Ruy repertoire to any player of the Black pieces with a positional bent.  In addition to the flexibility such a repertoire provides, I think that many of the variations are extremely solid and provide a positionally-oriented player of the Black pieces with a wealth of opportunities to outplay his or her opponent.  Of all the opening complexes I have played with either the White or Black pieces, the Black side of 1.e4 e5 is probably the opening in which I have scored most heavily against weaker players while doing better than expected against stronger players.  And if you are thinking about taking up the Black side of 1.e4 e5, Davies' book is an excellent place to start (I would recommend Flear's "The Ruy Lopez Main Line" as a supplement).  For those who might want supplemental explanatory material about Ruy Lopez positions and how to play them, "Mastering the Spanish With the Read and Play Method" by Daniel King and Pietro Ponzetto (Batsford/Henry Holt 1993/1994) is a really unique, non-theorectical opening book on Ruy Lopez pawn structures which you might still be able to find online and which contains the best general explanations of Ruy Lopez positions I have seen.

Just added the following to the Modern Steinitz, but maybe it should be here too...

When reading Davies book I've just come to the DERL, where he recommends a somewhat less practical move without alternatives i.e. a knight sac for 2 pawns (5.0-0 Be7 6.Bxc6 dxc6 7.Nc3 Bg4 8.h3 Bh5 9.g4 Nxg4)  which seems slightly dodgy to me, as black might need to take a perpetual early on in some side lines. Or is this really the reason why the DERL has fallen out of favour??

Actually, the DERL looks quite interesting to me and I don't see why it's so much less popular than 4.Bxc6, as compared to the Exchange variation white has gained 0-0 while black's Be7 needs to move again to cover e5. And then d4 seems to have some more bite than in the exchange.

Anyway, I consulted Keres and he mentions that black can transpose to the Modern Steinitz with 6...bxc6 7.Re1 d6 - which looks like the more fighting choice to me. So knowing something about the Modern Steinitz can be quite useful even if you usually prefer the other main lines...

Fernando Semprun in the Fascinating King's Gambit threat remarked that he thought the Davies e4,e5 was rather poor. 
I myself wouldnt go that far, even if I my repertoire choices are rather different in the e4,e5 system.
I have remarked upon this in a question on the spanish exchange variation. And in an italian thread I have pointed out a howler of an analysis-error by Davies copying/following the same error in Emss e4,e5 book.
My overall impression of the book is that it's pretty decent even though he has not convinced me to change my repertoire.
I would like to now if there are others who have a critical opinion on this book