It's quite short and set out in a question and answers style. The thing is, it very gently and very subtlety bends your mind around to the Lisp way of thinking. After reading this, SICP seems so much more accessible.
There are also two follow on books - The Reasoned Schemer and The Seasoned Schemer which are again quite short and use the same interactive approach. I haven't had chance to read them yet but they get rave reviews on Amazon