I enjoyed reading 'Concrete Abstractions', and it's usually very cheap if you buy it used on Amazon (I think I paid about $5.00 for a copy that was like new.)

I disagree with the recommendation for 'Simply Scheme', though. I didn't learn much from it, and I was disappointed with the amount of code in it that was non-standard stuff created by the authors purely for pedagogical purposes. I would personally recommend spending more time on HTDP or SICP instead of reading 'Simply Scheme.'