I helped work on the 2nd edition of this book, which I think is still fairly relevant: https://amzn.to/2IB9OyR
Although even that is not super recent.
Tcl's C API is very pleasant to work with, and its C code is a good read - except for the regex stuff, which are the first few files you'd see in an alphabetical listing.
I actually like Tcl quite a bit even if I don't use it much these days:
http://www.amazon.com/Tcl-Toolkit-2nd-John-Ousterhout/dp/032... (See the cover for all of the authors' names :-)