Hello,
I got to looking at the code in the booklibrary.php file and started thinking about the fatal error message I was getting. It was saying userGID was attempting to redeclare in booklibrary.php, as well as, cb.acl.php from Community Builder.
I don't know if it's the best fix possible, but I decided to rename the userGID function in the booklibrary.php file to userGID2. It seems to be working perfectly. I don't know if it has had any negative repercussions, but as of this point, it's working terrifically!