Most serious books about Mauritania are in French. English-language options are limited but a few are worth knowing about.
Two Rivers: Travels in West Africa by Peter Hudson — funny, honest travel writing covering West Africa including Mauritania.
Impossible Journey by Michael Asher — Asher and his wife spent months at Chinguetti preparing for a camel crossing of the Sahara from west to east. One of the best accounts of the Mauritanian desert.
Historical Dictionary of Mauritania by Alfred G. Gerteiny — the standard reference work for those wanting historical depth.
The Children of Mauritania by Lauren Goodsmith — a photographic and cultural portrait.
For broader context, the Rough Guide to West Africa covers Mauritania adequately, and the Wikivoyage entry is one of the more reliable free online resources.