Previous: Online Documentation, Up: More SBCL Information
If you're interested in the development of the SBCL system itself, then subcribing to sbcl-devel is a good idea.
SBCL internals documentation – besides comments in the source – is currenly maitained as a wiki-like website: http://sbcl-internals.cliki.net/.
Some low-level information describing the programming details of the conversion from CMUCL to SBCL is available in the doc/FOR-CMUCL-DEVELOPERS file in the SBCL distribution, though it is not installed by default.