Exercise for Pregnancy and Childbirth by Eileen Brayshaw is very good - informative and practical.
Some Chapters available if you copy and paste this link:
http://books.google.com/books?id=TIUaqqpjchIC&dq=eileen+brayshaw+exercise+ pregnancy&printsec=frontcover&source=web&ots=y4eF9 6RCmL&sig=T3d9S9WsmDVI15nyJGngziHJIkU[/URL]