How to Design Programs is a conventional college textbook. Felleisen and the PLT Group have a specific pedagogy developed during his decades as as educators with a deep interest in teaching computing [not just computing in general].

Realm of Racket is designed more or less as a self paced tutorial. It is in the tradition of Land of Lisp. It is targeted at a broader and more general audience.

I haven't read either cover-to-cover, but from the chapters I've read, Realm of Racket is a lot more conversational. It feels slightly gimmicky with the emphasis on games. Also Realm assumes you have some programming experience in other languages.

How to Design Programs starts at the very beginning and has a lot more principled of an approach, focusing on FP and programming as calculation of values. It's by far my favourite introductory programming text.