A reader on the Chess Publishing forum thinks that in Lesson 12 after the moves 1.Nf3 f5 2.d3 Nc6 3.d4!? e6 we should have dealt with the move 4.d5 (Dia). For the context I should mention that this position is discussed in Exercise 12 where the student is supposed to analyse the line independently, then check his analysis with a computer program and finally consult our suggested solution.
To my surprise I see that 4.d5 was originally suggested by a poster I have reason to believe is a fairly strong player. Yet I humbly disagree and assume the move was suggested somewhat light-heartedly - possibly for the fun of playing 2.d3, 3.d4 and 4.d5. That kind of fun can occasionally work well in closed positions. But in this case the Dutch leads to relatively open play.
In my opinion 4.d5 might reasonably have been discussed in some detail if it had belonged in an earlier lesson. However, after having dealt in some details with the comparable line 1.d4 e6 2.Nf3 f5 3.d5 (Dia) in Lesson 9, (where we opine that 3.d5 is unlikely to be a threat to Black's opening as it violates basic opening principles) I honestly don't think that this should be necessary. Black's extra tempo somewhat reduces his number of options but should not at all be difficult to use in a positive way.
Exercise 9 dealt with the highly tactical line 3...exd5 4.Qxd5 d6 5.Ng5 Qe7 6.Nxh7 (Dia)
Here Black has an important choice between 6...c6 7.Qb3 Rxh7 8.Qxg8 Rh4 as in an interesting game by Miles and 6...Rxh7 7.Qxg8 Rh4 8.Qb3 Nc6! as in a more recent game by Jussupow.
Having studied these positions it should be clear to any student that 4.d5 is, frankly speaking, a quite poor move. The main point is that after 3./4. d5 Black has the option to open the centre, reaching a kind of position where the pawn structure and even the number of pawns is of less importance than piece activity and direct tactical threats. As a result Black, as any analysis engine can confirm, has not one but a number of ways to reach a satisfactory position.
a) Rybka 3 likes the gambit line 5...Nf6!? 6.Qxf5 d5 7.Qd3 Bc5 and thinks that Black has full compensation. I am not sure that all programs will agree as some are very materialistic but most humans will find that Black has clearly the better practical chances here.
b) If you don't like sacrificing pawns 5...Bb4+ 6.c3 Nge7 7.Qd3 Bc5 is a simple way to complete development and reach a fully satisfactory position. Rybka considers that White is very slightly better here but I cannot really see why.
c) 5...d6 probably is even stronger. Black threatens to develop with tempo on White's exposed queen and 6.Ng5 (which is critical in the parallel position with Black's knight still on b8) is meaningless for at least two reasons:
c1) 6...Qf6 looks strong as the consistent 7.Nxh7? (Dia) loses to the slightly surprising 7...Qd4!.
c2) For practical purposes 6...Qe7 (Dia) may be even stronger as White may be tempted into 7.Nxh7 (what else?) 7...Be6 8.Qb5 0–0–0 9.Nxf8 Rxf8 when Black has more than compensation for his pawn. Rybka says '=+ (-0.34)' at search depth 14 but I suspect that White is lost.
White obviously should look for alternative 6th moves, but then Black will follow up with 6...Nf6 and e.g. 7...g6 and a very comfortable Leningrad position.
This analysis is based on my own evaluations, supported by some Rybka input, and I have not consulted my co-author who was the book's analytical authority.