JustAFish: I just picked up a copy of his "It's your move: Tough Puzzles" and am enjoying it greatly. The puzzles are mainly of a positional nature- similar, though not entirely like, those in Silman's "Reassess your chess workbook"- but there are a few mainly tactical ones thrown in. As advertised the puzzles are quite tough. The format of the book is interesting. He gives you a position, a brief assessment of the position, and then asks which of 6 fictional players' plans you most agree with. You are then given points based on which plan you picked. Most puzzles give partial credit for one or more of the non-favored plans, but the most points go toward the actual continuation seen in the game.
I've found it most fruitful to first try to come up with my own suite of plans before looking at those of Ward's fictional kibitzers- that way, I can assess my ability to come up with ideas in the first place. I've docked myself points for correct choices that I didn't independently conceive of.
As a chessplayer, one of the most difficult things I've found is not coming up with plans, but deciding between several appealing ideas. This book tests ones ability to assess the quality of a plan. Halfway through the book, I've found myself much more willing to try to "prove" plans, instead of simply assuming that any plan is better than no plan.
Some may quibble that a number of Ward's puzzles are rather subjective in nature, but I would argue that the purpose of picking up a chess book is to learn, not to prove something about yourself. I think I've learned some good habits.