Because Brittany is in France, a working knowledge of French is recommended.
Most books and methods assume the learner will speak French.
One of the few English online resources, Kervarker provides a basic learning curriculum based on “Ni a gomz brezhoneg” by Mark Kerrain
Ofis Public Ar Brezhoneg
The official site of the Breton Language, it provides links and information to events, courses and services to help learners.
Note that the site is only available in French and Breton
A small primer on Breton on Omniglot.com along with some pronunciation help and a wonderful video on the importance of Breton to Bretons.
Books for learners