I did read the warning at the top, but the update said it was working and deployed, so I went ahead and made a book.
I can't choose a download format (greyed out), so I can't d/l a pdf (also greyed out).
And I can't save all my work to my user location of User:Gemlog/Books/ nor to https://en.wikipedia.org/w/index.php?title=Special:PrefixIndex&prefix=Book:
Both produce an API error.
[Xb@-swpAIC4AALGGnJMAAAAS] 2019-11-04 06:05:39: Fatal exception of type "ApiUsageException"
I can, of course, give money to PediaPress. That link works perfectly and the books look amazing.
It would be a wonderful thing if the pdf worked like the July 2019 note says though...