Best book for me has been this forum!! I purchased Understanding Exposure after about 3 months of reading stuff on this forum. Flipped through it a few times and concluded that it has nothing to offer, nothing that I had not already learned from this forum. There's just an incredible amount of information here, it is free and most importantly, it isn't diluted. And by that I mean that you read through threads, land on stuff you don't understand and do a search and you get a straight to the point summary of exactly what you want to know. And if you can't find it, you ask and people overload you with information, a lot of time it is more than you need. I think that this source, POTN, is more than any book can be - it covers all subjects you can think of and most importantly, it is information that is tested by fellow photo-buffs and scrutinized to death if it isn't 100% accurate. What more can you ask for?