My favorite thing having finally taken a dive into lisp and SICP, is not that it is very friendly to functional. It is that it is very friendly to showing how it all works.

My favorite section is where they go over making a constraint based system that will either calculate degrees F or degrees C. Or validate that the two given values are accurate. All depending on what you have entered. And this is done from the ground up. No hidden magic of the system has to support you, by and large. (Not strictly true, as this does make use of GC and such throughout.)

If you hadn't seen it, https://www.infoq.com/presentations/We-Really-Dont-Know-How-... is a great video showing Sussman's style. He also showcases a program they wrote that could solve circuits based on this general idea.