These are not exhaustive bibliographies, by any means. I’ve tried to list standard sources that are relatively easy to find and fairly reliable. There are lots of excellent books I haven’t listed, and lots I don’t know about. Every book here that is specifically Celtic in scope is by an author who reads at least one of the Celtic languages. All of the books have a brief annotation, and I’ve linked longer reviews of some of the books.
Most of these books are available in paperback. Many of these are available at your local independent or national chain bookstore, at Barnes and Noble.com, or at the specialist book seller for Celtic scholars and students, Books for Scholars, or via the international UK/US scholarly book store Oxbow Book/David Brown Books, also listed on the Where to Buy Books FAQ.
I have included links to the Barnes and Noble and Powell’s catalog pages should you want to order a book online (I am an affiliate of both sites). In many cases you’ll find it is cheaper to purchase books from your local independent book seller. I’ve also begun posting a few longer book reviews.