From a bug report... some
special case handling for polymorph of spellbooks never worked as intended.
It's possible to polymorph a spellbook, use it to learn a new spell, and
then repeat as many times as you like unless/until you run out of polymorph
magic or the small chance of "object shuddering" causes it to be destroyed.
Polymorph was incrementing the book's ``number of times read'' field with
the intent that it would fade to blank after being read 3 times (which
turns out to the 4 times since the check is actually for re-reading 3 times
after the first). That didn't work because the spestudied field was ignored
when learning a new spell, only checked when relearning a known spell.
Now it will be checked when learning a new spell, and also the book
tweaking during polymorph is slightly more elaborate. If you happen to
get a blank book during the item selection, it will have a read counter
of 0 and can be re-polymorphed into something readable. But if you get
some other book, its read counter will be set to one greater than than the
original book's (same as before). And then the counter will be checked
to see if it has gone over the limit, in which case the book will be made
blank and its counter will be reset to a random value. Re-polymorphing
that blank book again has 1/4 chance apiece among the following cases
book gets blanked again; goto step 1...
book is non-blank but too faint to read; reading attempt will fail
book can be read normally and then re-read once
book can be read normally and then re-read twice
which is more inline with the intent of the original special case code.
It's actually slightly nastier since you'll occasionally get a book for a
spell you don't know yet but then not be able to learn it from that book.
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