A number of people have said bad things about Vorhaus' "Killer Poker" series on this forum. I don't think these are bad books, but they're books that are useful for a very specific audience and not at all useful for others. Many times people say "book X sucks" when what they ought to say is "book X wasn't of any use to me". These are two very different concepts that often get confused on this forum.
If you're playing weak tight or having discipline problems then I think "Killer Poker" may help you achieve more focus and play more aggressively. If these aren't your problems then these books probably won't be useful to you. I have full reviews of some of JV's books here:
http://www.jetcafe.org/~npc/reviews/...ler_poker.html
http://www.jetcafe.org/~npc/reviews/...er_online.html
http://www.jetcafe.org/~npc/reviews/...ker_night.html
One other note. It's my opinion that Vorhaus is probably the most skilled writer of all the strategic poker book authors. Some very skilled writers have written poker entertainment books, and several strategic poker writers are quite competent, but I think JV is the best of this crowd. Even if his books don't help me, I enjoy reading them.