Marc Staniszewski wrote a book "Mantellas" many years ago. Although his books are sort of a mainstays, they're quite outdated now (plus they're really expensive). I would recommend some of the links that Lynn posted in the Dart Frog section. I'd link that post but I don't know how.

Check out this website too. Devin Edmonds (the author) is now running a mantella captive breeding program in Madagascar.
Articles About Mantella Frogs

Cheers,
Alex