I compared the chapter about 4... Bc5 5.Nf5 and 5.Nb3 (extract availabe on website) with the most recent book I own on this subject, Play 1.e4 e5! by Davies from Everyman Chess too. The latter is a repertoire book for Black from 2005, with one full game (game 52) of 3 pages covering both these variations (15 pages for YD&RP). You cloud expect the comparison to be unfair for Davies, but it's quite the opposite! Two exemples:

1.e4 e5 2.Nf3 Nc6 3.d4 exd4 4.Nxd4 Bc5 5.Nf5
Here Davies recommands 5...d6!?, which is unmentionned by YD&RP. It's not played often, so no big deal. But he justifies his choice by mentionning the main line 5...d5 6.Nxg7 Kf8 7.Nh5 Qh4 8.Ng3 Nf6, with the move 9.Bb5!? bothering him. Whether or not it's annoying for Black (I rather think Black can claim equalitiy or more in this position), I'm disapointed that this move is unmentionned by YD&RP. They discuss 9.Be2, 9.exd5 and even 9.b4 (what's the point?), but not a word about 9.Bb5!? which certainly deserves more consideration than the 2 latter moves.
It's true that 5.Nf5 isn't play at high level, so relatievely poor coverage of this line may be acceptable. But keep in mind that 5.Nf5 is seen occasionaly at amateur level!

1.e4 e5 2.Nf3 Nc6 3.d4 exd4 4.Nxd4 Bc5 5.Nb3 Bb6 6. Qe2!? is on the other hand more topical at high level, with Ivanchuk and Carlsen playing it during the last decade. YD&RP end their discussion by claiming that White may currently be slightly better in this variation. If we consider Davies' recommandation, he goes for 6...a5, forcing White to play 7.a4. Hence, White's main idea (going for opposite-side castling) is not so effective or more dangerous for him/her. Davies and YD&RP mention the same game (Novikov-Aleksandrov from 2005) and agree that Black has a good game. If they are consistent, YD&RP should have an improvement for White. Indeed, they just mention a move along the varaition, after 7.a4 Nge7 8.Nc3 Nb4, 9.Nb5!? (instead of 9.Bg5) may be critical. They stop here... The next moves could well be 9...0-0 10.c3 Nbc6 11.Be3 Bxe3 12.Qxe3 with differet options for Black: I particularly like 12...f5!? and I doubt that White can claim an advantage, but I would be interested to have the expert opinion of YD&RP about it. Do they think White is better?

I'm slightly disapointed by this comparison. Or impressed by Davies' work! Of course, this chapter is probably not the most important one in the Scotch, and I'm waiting to see the whole book, that I hope to be better than this chapter.

I agree, if it's 400 pages, "monster" apt.

well, i would suspect they do consider Gustafsson's lines in their book, i would guess that would've found either and improvement or other lines that he didn't consider, since they have the benefit of hindsight and also a good 3/4 months of games played.
I think this book will be a monster book, precisely because Palliser in particular is very very very very very very thorough very. Dembo is also quite thorough, in fact i thought originally that Dembo was supposed to be writing this book entirely by herself but since it proved too exhaustive (it is a very big opening to cover) she enlisted pallisers' help.
And yes, Dembo and Palliser are probs they would be the most diligent which means precisely that this book will be packed with theory. Lucky i don't play the scotch as white.

I am curious if they have considered Gustafsson's lines from both his DVD and his blog (where he updated his DVD lines with the recent Tata Steel tournament).