I highly recommend this book. I used to pick this up from time to time when I didn't understand a concept from HTDP. Their explanation of the lambda as translated from Math to Programming really made me get it. One of my first joyous moments reading Scheme.
Or you could just take the MIT OpenCourseWare class <a href="http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/6-001Spring-2005/CourseHome/index.htm" rel="nofollow">http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Compute...</a> (uses Scheme also)