Advanced: vop - calls
SBCL has an extremely sophisticated facility called a VOP (Virtual Operation) that allows you to write assembly with s-expressions and integrate the code into the compiler and runtime. While this book is not only about SBCL, I cannot find a good guide on writing those, in contrast to other sbcl-specific feature. So, maybe consider making this a chapter?
Unfortunately, I cannot do this at all. Even more, I kinda want to read this somewhere myself)
https://www.pvk.ca/Blog/2014/03/15/sbcl-the-ultimate-assembly-code-breadboard/
Also see other posts from the same author.
@Sokolov57 Personally I have never used this.
I am not sure if it is a good idea to add implementation SBCL-specific content, considering SBCL documents are pretty good IMHO.