ex : (= e ) (prep. + abl.) out of, from within, from / on account of.

liber libri : book.


Ergo, "out of book(s)" or "from within book(s)" or "from book(s)".

Merriam-Webster:

Main Entry: ex
Pronunciation: (')eks
Function: preposition
Etymology: Latin
1 : out of : FROM: as a : from a specified place or source b : from a specified dam
2 : free from : WITHOUT: as a : without an indicated value or right -- used especially of securities b : free of charges precedent to removal from the specified place with purchaser to provide means of subsequent transportation

Hence, "(formerly) from book(s)", as in "ex-Californian."