How secure is a book derived key and what can be done to strenghten it?
The key selection must always be sufficiently random. 

The key can never be considered reliably secure if the selection is simple ("Book A, page B") since the number of possibilities is necessarily too low.

Considering poems in Afrikaans has been cracked it's a necessity to NOT rely on a published book being unknown to the adversary. 

Secure book based ciphers often include a list of references to individual books, individual pages, and different paragraphs in turn. In this case it is the sum of many independent unpredictable selections that provide the security a key needs.