Thank you Jean for pointing me to those instructions. They are very clear. I followed them to the letter. I used plain cmd -- even in admin mode -- and edited the path to where my LilyPond 'bin' is located. I closed/re-opened cmd. I even restarted the computer. I still get the same message that lilypond-book is not recognized. I even tried using the long version:
python C:\Users\me\lilypond-2.25.8\bin\lilypond-book.py myfile.ly
...edited for my installation, of course, and got:
'H:python' is not recognized as an internal or external command,
operable program or batch file.
(Obviously, "me" needs to be replaced with your user name, here and in the linked instructions.)
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