How secure is a book derived key and what can be done to strenghten it? If you have a manual encryption system where the keying is done by extracting a key from a book, a major weakness is that an adversary can search trough all existing books to find a perfect Match and thus the key. But isn't that very theoretical since not all books can be accessed by every one on the internet? 

What can be done to strenghten such a book derived key?
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.