What books would you recommend to someone regarding Ba Gua history and philosophy?
The best information on Ba Gua Zhang in English is that printed in the Pa Kua Chang Journals. Some back issues are still in print and are available from Plum Flower Press. I haven't seen any books in English that I think are too good so far.
All of the back issue of the Pa Kua Chang Journal are now available on one CD-ROM from Plum Flower Press (www.plumflower.com).