The Scheme Programming Language by R. K. Dybvig—you’ll notice his name is also on R5RS —describes all the features of standard Scheme. SICP won’t be using every feature of the language, so you may find this book useful if you use Scheme outside the project. The third edition is available online.
An excellent book for getting the feel of the language is The Little Schemer by Friedman and Felleisen. It’s quite different from any other programming text I’ve read (although the cover reminds me of The Fortran Colouring Book by Roger Kaufman), but it’s very useful. In the foreward, Gerald Sussman describes it as something that can “perform the same service that Hanon’s finger exercises or Czerny’s piano studies perform for the student of piano.” There’s no other way to describe it without showing it to you.
Peter Seibel’s book is also available online. This book looks particularly interesting since it contains several practical Lisp applications—rather than the more academic ones we find in The Wizard Book.
Paul Graham’s book is really a collection of essays, many of which promote Lisp in one way or another. They’ve certainly convinced me the language is worth exploring. Here’s the book’s homepage.