Well, I managed to break things completely.

Note one -- like an idiot -- I didn't do a sql dump prior to
messing around with things. (you know where this is heading)

Note two -- there were a lot of older files in the 'books'
directory, so I moved it out of the way. Copied over the
books750RC4 files, updated the bk-config.php file, regenerated
and updated.

After a bunch of messing around, where all I was getting was
the books module not found, I finally got;

Fatal error: Call to a member function PO_RecordCount() on a
non-object in /PATH/modules/books/admin.php on line 1601

or

Fatal error: Call to a member function PO_RecordCount() on a
non-object in /PATH/bob/modules/books/bk-navigation.php on line 83

Fortunately, there hasn't been many changes lately, so my
SQL dump from last week should be fine.