The first in depth book on Chickamauga was Glenn Tucker's book "Chickamauga",it has been around for many years so you should be able to buy it used for a dollar or two.
Turchin'so book is also old,old in public domain as is Archibald Gracie'so book but his is mostly on Horseshoe Ridge.Willich's also has a book in public domain.
The old Newspaper the National Tribune has about 250 articles on the battle with the best being a three week series that was written by Wm.S.Rosecrans,his only record of the battle outside of the O.R.'s.
James Longstreet also has a good article also.
The 5 Powell books is really the way to go though with Vol.III having a great appendix of resources of books,article and websites to manuscripts and diaries.