Sketchy LISP Vol. II – Reference is available. From the introduction:
Sketchy is an interpreter for a purely applicative dialect of Scheme. It may be considered an implementation of pure LISP plus global definitions, first-class continuations and input/output functions. Like its first volume, this part focuses on the functional aspects of the Scheme language. While the first volume provides a step-by-step introduction to Scheme in general, this part describes the Sketchy subset as a formal system for re-writing terms. It introduces semi-formal definitions for Scheme data, programs, a small set of primitive functions, and a set of rules for reducing purely applicative expressions to normal forms.
As you can see, this second volume contains more advanced stuff, and gives you yet another opportunity of digging deeper into abstract thinking. I just ordered my copy, but the full text is available online if you’d rather save twelve bucks.