I recently was reinvestigating the usage of Scheme as an extension language for an engine that I was developing. I used to use LISP a lot, especially in my previous life as a fire suppression engineer when I used AutoLISP within AutoCAD to create macros. Scheme is a dialect of LISP, so it retains some of the syntax, but Scheme is minimalistic compared to LISP. Scheme has a well-defined, small specification while the LISP specification is considerably larger and more comprehensive. So, when it came time to do some examples in Scheme, it was very familiar to me, outside some of the transitional items.
There are many Scheme implementations (see the list of Scheme implementations), but GNU Guile was built for extending software. Plus, it was already on my Linux system, so it was a good place to start.