Reddit reviews Arthur in Medieval Welsh Literature (University of Wales Press - Writers of Wales)

Thanks for asking!

I think the main thing is to avoid Llewellyn publications. Even the Arthurian scholar R.S. Loomis's books are extremely outdated but are probably easier to find on the market as they can serve as "proof" behind the new age books.

I'd recommend anything by Kenneth Jackson, O.J. Padel (great Arthurian source!), Proinsias MacCana, Rachel Bromwich... Miranda Green also comes to mind, but iirc it's her work on the Green Man that I would take with a grain of salt.

Hope that helps!

edit to add: A.O.H.Jarman and Patrick Sims-Williams too!

edit2 for personal favourites:

After Rome by Thomas Charles-Edwards Note: this is on "post Roman Britain" not "dark age Britain"

Celtic Mythology by Proinsias MacCana. Some of the research is outdated but it is still considered, to this day, an awesome go-to book (pretty pictures!).

Arthur in Medieval Welsh Literature by O.J. Padel. Hands down, the best book I've come across on Arthurian origins and it's extremely accessible for the average reader. It's quite short so anyone interested in this subject should absolutely pick this one up. The prices in this link are ridiculous though. (Great news - this one will be reissued in August 2013!)