T.S.
Mercer Mayer's Little Critter book, "The New Baby" was our favorite, short, sweet and to the point.
Also, make sure you don't make TOO big of a deal out of the new baby, it can backfire. Keep the baby talk to a minimum and try to keep your son's life as normal as possible. Even though we parents see a new addition to the family as a huge deal (obviously!) young children just don't care that much, and they can easily be stressed out if you turn it into a big thing.