I don't think a book is "not language agnostic" because two particular languages aren't very good at it. C's lack of closure support is unique among any language still in use, for a variety of reasons. Java... was Java. If you weren't there you may not remember or understand, but at the time OO was at its most dogmatic and either it was an object or it was not included. Anything you could do with a closure could be done with an anonymous inner class. (Which is, technically, true. But the amplification factor on the effort and the sheer quantity of text involved to do so in Java is astonishing.) You can hear some of the echos of this in the older Ruby rhetoric, though I haven't heard it in a while.

Any other general-purpose language you'd be inclined to pick up right now would be fine.

IIRC, you end up implementing a scheme, which you can do nearly in the language of your choice, then the book covers that language you are implementing.