I’m not sure how much of it is still relevant to SBCL since the fork was quite a while back, but the CMUCL user manual is a great read that describes how type inference and many other things work. https://www.cs.utexas.edu/users/jared/Milawa/Support/cmucl64...
Probably still relevant given how SBCL's user guide says (in the only section that includes mention of type inferencing):
"FIXME: The material in the CMUCL manual about getting good performance from the compiler should be reviewed, reformatted in Texinfo, lightly edited for SBCL, and substituted into this manual. In the meantime, the original CMUCL manual is still 95+% correct for the SBCL version of the Python compiler."
