lilypond-book isn't called by LaTeX, it's a preprocessor that you first have
to run on a .lytex file to get a .tex file, then you compile that .tex file
with TeX.
Consider lyLuaTeX, which is an alternative that runs inside the TeX process
directly, but requires LuaTeX. You can specify the path to the LilyPond
executable with \usepackage[program=/path/to/bin/lilypond]{lyluatex}.
See https://ctan.org/pkg/lyluatex
Fortunately for me, I'm using lualatex already, so that worked. I do indeed lose point-and-click, but it's good to know that if frescobaldi stops working I at least have a backup. Thanks for the help!
N. Andrew Walsh
er/ihn/ihm/sein | he/him/his
Berlin