"- Does your hat still fit?""- Of course it does. After you have worked through the definition of the Y combinator, nothing will ever affect your hat size again, not even an attempt to understand the difference between Y and Y!"
Well, the quote pretty much nails it: The Seasoned Schemer, also by Daniel P. Friedman and Matthias Felleisen, is a very good complement for the first book, and as recommended as "The Little Schemer".
Except that... I don't agree with their "nothing will ever affect your hat size again": I think there are lots of "head expanders" out there waiting to be read.
But I'm taking a break before the third one: it's healthy to keep the same hat size sometimes :-)