There are so many childfree books I adore, but Jennifer L. Shawne's Baby Not On Board really may be my favorite, not only for its entertainment value but for the way it breaks down the reasons not to have children into such simple and humorous yet truthful forms. Even my husband found this one amusing as he read it over my shoulder, and he's not usually one to get too into this stuff (though he doesn't want kids any more than I do, he doesn't really care about the whole childfree community thing.) Baby Not On Board is a lighthearted take on the childfree subject, but as is so often the case, there's a lot of truth behind the humor.
If you ever need to be reminded of what could happen to your budget, home, dinner, or travels if you had kids, this is a good way to get that reminder and a laugh at the same time. I adore the cartoon drawings it includes of "Your Home" and "Your Home with Kids," "Your Dinner" and "Your Dinner with Kids," and so on. Since Ms. Shawne and her husband, like me and my husband, live in a large city and are fairly young, her life without kids is actually a lot like my life, and she does a great job of pointing out just how great that life can be and how deeply having kids could screw it up (my favorite part is her chronology of a typical Saturday in the life without kids, and then with them, which is funny yet has the chilling ring of truth.) The book also deals with such topics as "coming out" to friends and family, and has a humorous chapter on dealing with OPCs (Other People's Children).
In short, this is a great read for sometime when you want a childfree book that isn't too serious. The author, Jennifer L. Shawne, also has a blog by the same name as her book, which I really enjoy and which you can find under my links section.