Im going with Amy, 'Conception, Pregnancy and Birth' was the best $50 i ever spent on a book. Very helpful, loads of information, pictures and ideas. DH and I loved reading it together.

I also have 'What to expect when you're expecting' and its a combined with 'What to eat when you're expecting', was a good book off MIL, dont refer to it all that often but now and then i do as i think its goes more in deepth with answers i want to know.