There’s quite a lot more stuff in Common Lisp that in XLISP. You’ll probably get quite frustrated if you use a book about Common Lisp as a language reference, but it will probably still be an excellent reference for other things, such as how to write in a “Lisp-like” way.
When I started using Nyquist, there was very little documentation. There was a Nyquist manual (similar to the current manual but less complete and less accurate), a brief introduction to XLISP by Edgar-rft, and (I think version 1 of) the XLISP manual by David Michael Betz, and the original version of this: http://audacityteam.org/help/nyquist There were also a few Nyquist plug-ins shipped with Audacity that could be used as examples, but they were not written by experienced LISP programmers so were not particularly good examples of “the right way” to write.
Since then, those documents have all been updated and expanded, and Edgar-rft wrote this “Nyquist Plug-ins Reference”: http://wiki.audacityteam.org/wiki/Nyquist_Plug-ins_Reference
My list of on-line documentation:
http://wiki.audacityteam.org/wiki/Nyquist_Plug-ins_Reference
http://www.audacity-forum.de/download/edgar/nyquist/nyquist-doc/xlisp/xlisp-index.htm
http://www.audacity-forum.de/download/edgar/nyquist/nyquist-doc/manual/home.html
http://www.cs.cmu.edu/~rbd/doc/nyquist/indx.html
http://dept-info.labri.fr/~idurand/enseignement/lst-info/PFS/Common/Strandh-Tutorial/indentation.html
There is also Robert’s “Little Helper” reference: https://forum.audacityteam.org/t/nyquist-reference-in-a-nut-shell/26911/1