"The Art and Craft of Problem Solving" is widely used here in the Malaysian National Team in preparation for the IMO.
I'm quite familiar with the book, though I haven't fully read and understood all of its contents to make any proper judgement on its utility. The national team's main trainer did recommend me this book, adding that he has been using it for more than 10 years now.
I believe that it all depends on your real objective: are you preparing for the IMO, or do you merely want to inculcate fundamental problem-solving capabilities for use in other areas? Although I won't say for certain that this is the best book available, do keep in mind that it's a resourceful text written by one of the most successful US IMO coaches (he trained the "dream team" in 1994; all 6 members of the US IMO team scored full 42 points).
If you are preparing for a mathematical Olympiad of any kind, I would also recommend books by the Art of Problem Solving community, and a couple by Titu Andreescu / Zuming Feng.