Hal Leonard Djembe by Ivano Torre

Ivano Torre combines traditional Western musical notation and understanding with the techniques used while playing the djembe that were developed millenia ago in Africa. The end result is Djembe: Method for the Technical and Creative Development, which contains nearly 100 pages of gradually advancing exercises intended to make you completely comfortable using any technique on any beat. Also, it's written simultaneously in English and Italian, so there's that.