The Little Schemer book is also a deceptively deep introduction that is a good preparation for On Lisp. If you work your way through it, you'll really understan

The Little Schemer book is also a deceptively deep introduction that is a good preparation for On Lisp. If you work your way through it, you'll really understand functions that make functions, which is a good foundation before diving into On Lisp (and, of course, as a bonus, you'll have written an implementation of the famous Y combinator:)).