A treatise on ants, written by Bert Holldobler and Edward O. Wilson. It won the Pulitzer Prize in 1991.
This is a big (732 page), extensive, and wonderfully illustrated book on all aspects of ants. Although it contains all the data an entomologist might want, it is readable for the lay public.
It's a great book for browsing, and makes a nice coffee table books. Even if you'd never buy it, it's worth checking out of the public library.