A word about the 'Jackal Attack' re: a previous post.
...and where is The Jackal?   
 
Yes, there are many lines/systems not yet covered by any book...maybe not "so dangerous" (in the sense given by editor of books about the "dangerous" way) 
 
Thanks to chesspublishing and NiC (see Glenn Flear review in one NiC Survey about the Jackal) to publish about unusual systems. 

It may be worth mentioning that since Flear's 'Way of the Jackal' article [NIC '04], this opening gambit  - 1.e4 e6 2.Nf3 d5 3.Nc3 Nf6 4.e5 Nfd7 5.d4 c5 6.Bg5 - ('The Jackal Attack') has been used successfully several times by strong players of up to and including IM strength. 
Leaving aside that improvements in the main line have since been found and analysed, 
the original 'Day of the Jackal' article [ Skelton; NIC Yearbook:Vol 61] has proven largely accurate in predicting where Black will have most problems (and successes) in this line.

Flear's opinions on the Jackal was partly based on the above NIC Yearbook article, but mainly on a secondary source; a  monograph which gave a preliminary survey of the various lines springing from the above opening sequence. 
As Flear pointed out,  there were various 'gaps' in the monograph's analysis. But it was precisely for this reason that both it and the NIC 61 article had tended to err on the cautious side when it came to evaluating the efficacy of the gambit.
Mr Flear, quoting the analysis in both sources, seems to have interpreted that caution a little bit too literally and as a consequence reached his conclusion that the Jackal Attack shouldn't 'trouble the big guys'. 
As an example, he goes on to offer his own independent analysis  to illustrate his point and to show how 'unrealistic' the plot of the Jackal Attack really is. 
He does not make a very convincing case: 
 
[1.e4 e6 2.Nf3 d5 3.Nc3 Nf6 4.e5 Nfd7 5.d4 c5 6.Bg5 Qa5

“Seems to be comfortable and avoids complications.”-  [Flear NIC ‘Way of the Jackal’]

7.Bb5 a6 8.Bd7 Bd7 9.O-O Nc6 10.Re1 cd 11.Nd4 Nd4 12.Qd4 Bc5 13.Qg4

['about equal' - Skelton;'The Jackal Attack' monograph '02; e-book '07]. 

Flear now comes up with...
 
13...Qb4

Clearly impressed with his find, he awards this as [!- Flear] and points out that the attack on the 'b' pawn now forces White to trade queens, after which Black has slightly the better of things.
He therefore gives White’s best as 14.Qb4 Bb4 15.Bd2 Rc8 with a slight edge for Black and further notes that White cannot cut across this plan by playing 14.Bf4 since after 14…Qb2 15.Qg7 Rf8 16.Ne2 Bb5, Black has a still more tangible advantage. True; but basically irrelevant. It's not the detail, but Flear's concept that is dubious.
It might be worth quoting fron the original 'Day of the Jackal' article:
“White’s threats tend to multiply when Black captures the ‘poisoned’ pawn. The fact is that players of all standards will be tempted to take on b2.”- [Skelton; New In Chess Yearbook 61.]   
In fact White has two interesting moves here - neither of which the GM mentions - yet both can be extremely dangerous for Black. Moreover, few players worth their salt are likely to go in for the trade of Queens in this position.
14.Qh5!? This can lead to quite complex lines if Black follows through with the threat to the 'b' pawn. After 14...Qb2 15.Nd5 the approach of the knight should not be quickly dismissed; nor should the ensuing complications after Black refuses the piece.
Arguably still stronger  for White though is the solid 
14.Qf3 Again, capturing the 'b' pawn can be shown to lead Black into a bit of a quagmire. A more sensible alternative therefore is the natural 14...Bc6.

Thus, in his own attempts to show the transparency of the Jackal's 'plot' - and how easily it is refuted by the big guys - Flear unwittingly succeeds in demonstrating the opposite, and on both counts.  His  general assessment and opinions of the gambit must therefore be weighed in light of the above.

Attempts to honestly assess positions which are both untested in practice and unknown to theory, is an interesting but quite difficult task - especially if one is looking to avoid glib appraisals. This was why the original monograph analysis was certainly spartan in places. I try to assess such positions from a player's practical perspective. In this regard, as much more experienced analysists have pointed out, computer generated analysis has its limitations. For example after: 1.e4 e6 2.Nf3 d5 3.Nc3 Nf6 4.e5 Nfd7 5.d4 c5 6.Bg5 Qb6 7.dc Bc5 8.Qd2 Bf2 9.Qf2 Qb2 10.Kd2 Qa1 the silicon chip will tell that White is busted. Only human 'intuition' would persist with the analysis further to discover that in fact Black is somewhat struggling here - as subsequent games have shown.
I therefore stand over my original assessments. 
As for the 6...Qa5 line, as recommended by Flear: It is certainly playable for Black and avoids the 'complications' in the sharper lines; but Flear's move can be counter-productive to this aim and certainly offers no refutation of the Jackal.

In his defence, I should point out that the GM's observations on the more cosmetic aspects of the original monograph were actually quite fair and even useful: Anyone who obtained an early version can download a far more detailed (ebook) analysis of the gambit through the jackal attack website for free.  It is only Flear's analysis and the inferences he draws from it that I take issue with.
As for the gaps. Better a gap than a gaff.

Is the Jackal 'sound'? 
Many gambits probably aren't but still continue to win: Having lost to one, it is little comfort to be told afterwards that such and such a line is considered to be theoretically unsound.
A more appropriate question is therefore;
'Is the Jackal dangerous?' That's quite a different matter. 
A.S

I have just received the book from Amazon. I admit that in purchasing it I am more interested in the black lines rather than the white ones. My initial impression is very positive. 3...Nc6 looks like an interesting line against both 3. Nd2 and 3. Nc3. Watson claims that all the lines he gives are sound (which is not to say that they are risk-free) and he points out where he disagrees with Khalifman's analysis in his book on the Winawer (Vol 7 of that series). I must admit that I am very sceptical about a few of the lines he gives (for example the ones involving early h6 and early Bf8 in the Winawer- surely development means something). Some of the lines he gives though have been taken up by many strong players and could almost be considered main lines (eg. 3... Nc6 against the Tarrasch). Overall this volume seems to be on the par with the one on the Sicilian which was in its own right a very good book.

Anybody knows when the book is coming out?

Thks, TimS... I parsed the game and got the feeling, the gambit gave no advantage or initiative (5.Bd2 only gives the e4-pawn), but all experiments are interesting in the line

I am not too sure about it. Chapters 5 and 6 I have met on the board, with the former being a bit tricky, but the latter was no problem at all. Chapter 8 indeed is a nice surprise and chapter 10 I have read about before, assuming the idea is to exchange the white squared bishop. Chapter 3 looks the most dangerous, but I am unaware of any improvements for white and he certainly needs them as I dont think white has won in this line since 2000.

Where's the Haldane Hack?

Thanks Top! I will certainly have a look as I only play the M-B and I could use some deviations..

I would have thought the norm is to meet 5. ed with ...Qxd5 in the MacCutcheon.