I have been considering learning Latin with “Scanlon’s Latin Grammar.” It is liturgical Latin set up for reading the Missal and Breviary. Unfortuanately, I have not been able to find an answer key for it. Perhaps if you had it on line, there would be more interest in it and someone would start a group.
Textkit can only host public domain grammars, so if the book’s copyright is less than about 80 years old, it’s off limits.