Well, the gold standard in fitness pregnancy books is Dr. James Clapp's Exercising Through Your Pregnancy, but it is currently out of print and hard to find. It doesn't give you specific exercises to do, but it presents the most thorough and scientific explanation of how exercise affects a pregnant body and why it's good for mom and baby. It also is one of the few pregnancy/exercise books out there that allows for the possibility that very fit women also get pregnant and don't necessarily want to spend 9 months with just "light walking."
If you want specific exercises, a lot of people seem to have recommended Expecting Fitness by Brigitta Gallo. Sheila Watkins, the moderator of this forum, usually recommends a book called something like "Essential Exercises for the Childbearing Years," but I've been unable to find the exact name and author by searching the forum, so hopefully someone else will remember! Fitness magazine also has a book out called Pregnancy Fitness (I think) that I thought was pretty good.
But of course, the best resource is this forum! (How's that for an unsolicited plug?!) Good luck with your future family plans.
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