I've generally found, both with my own code and others' code, that `some-undocumented-special-form' is about as understandable as `some-undocumented-function' (and a similar equivalence for things that are documented).
That greatly depends on how principled (hence convenient) the mechanisms for redefining and/or extending the language are. Metaprogramming in Python has the annoying tendency to create a mess where the implementation details of different parts are tightly coupled to each other. But metaprogramming Racket, especially using syntax/parse, is very clean.
Interesting. I wish there as a Manning "Racket in Action" book. The problem with a lot of Lisp materials is that they show you the basic constructs and not how to best use them (like you talk about above).
On Lisp. PAIP, Common Lisp Recipes, Practical Common Lisp, Lisp Style and Design, AMOP, Object-Oriented Programming in Common Lisp: A Programmer's Guide to CLOS, ...
There are various Lisp books which go beyond the basic constructs.
