I thought it is to help to learn list manipulation. String is great but is a super coat. But I do agree this should not be the only book. In fact Piap is better.
The whole Common Lisp learning is hindered by suddenly you have a function that do magic. And nil does not help.
Really wish there is a lisp book learning using the kernel (lisp 1.5 which can be easily implemented using itself).
It's not a book, but there is Graham's short paper "The Roots of Lisp" - http://paulgraham.com/rootsoflisp.html which is something of this sort: you can more or less implement something like lisp 1.5 in CL or elisp etc. following this paper.
The whole Common Lisp learning is hindered by suddenly you have a function that do magic. And nil does not help.
Really wish there is a lisp book learning using the kernel (lisp 1.5 which can be easily implemented using itself).