Another great source is
International Bonsai magazine. Many of their issues are devoted to a single tree species and the back issues can be purchased at a very reasonable price.
Peter Adams' various out-of-print books may be harder to find, but they are excellent resources on specific species as well.
Just remember, if you happen to find his highly sought-after Maples book, who turned you on to it in the first place...
Best regards,
Old Mister