During my first pregnancy, this was one of the books I've constantly turned to as reference:

What to Expect When You're Expecting is probably on of the highest-sold books on pregnancy (in Amazon, it has already been reviewed by more than a thousand readers. It has received mixed reviews, some saying it's comprehensive and easy to digest, while others found it too alarmist, "raising everything that could possibly go wrong". Personally, I found the book very informative, and delivers what it promises in the title. I would agree though that there were parts that make pregnancy almost like a disease, and if you're the type of person who worry a lot, it may drive you into paranoia (or frequent-than-normal visits to the doc). I would still recommend getting hold of this book, but to not consider it as a pregnancy-bible of sorts.