Personaly i found abcl a bad experience.
Thoughts on ecl & clisp ?
sbcl works nice & fine. But i’t’s the only lisp implementation i know.
There are good books on racket-scheme & chez-cheme.
The only book i know for lisp is, “Common lisp , a gentle introduction to symbolic computing”.
Well, I was judging by its official page and Sourceforge activity, which seems negligible. However, Gitlab also does not seem very active (6 commits this year, despite lots of open bugs). The development is almost stalled. It is on life support, at best.