Everything there is Old and Out of Copywrite.
That’s why its available here and free. 😀
Here is the Mythology & Rites of the British Druids:
Published in 1807, it is something that could be included in the BDO Library.
To be honest, I tend to avoid anything written on the druids between the 18th and early 20th centuries unless it’s directly dealing with folklore and folk stories or genuine Celtic literature. Most of what was written about druids in that period was wild speculation, unreliable and filled with some pretty strange and, by now, debunked notions and ideas. I don’t think they’re very helpful at all and, worse still, I feel they can be counterproductive.