There are for at least Breton, Welsh and Irish some good books, but which language do you want to learn? I hope not just 'any celtic' one, because the cultures may share many things, they are not the same and for the languages counts the same thing. Dou you already know if you want to learn British Celtic or Gaelic? Manx is a Gaelic language, and alas, I don't know about any books for that. But I might be able to help you a little bit with british....
pokou
