SICP is more for college level students with some high level mathematics background (basic calculus) and tackles computer science ideas directly. HTDP (at least the first edition http://www.htdp.org/ ) was intended for beginners without as much math background except for algebra. I actually prefer HTDP over SICP as an introductory text in computer programming and the Scheme language in particular. It's a great read in how to approach programming problems. I was actually exposed to designing with contracts through this text before I was ever exposed to Eiffel. And it's approach to teaching recursion was very gentle and practical. While SICP is great in ideas and prose, it is in some ways too abstract. HTDP is more grounded and covers those details that SICP assumed you'd have. No harm in reading both though.