There is a major side channel attack on such ciphers, imaginatively called a premises search attack. Whilst you'd think that all the books in the world are at your disposal, in fact they're not. You have to have the book at some point, and so does your cipher text recipient. And they have to be word for word, page for page identical.
So if you're reporting on a local colony of wombles, you might encipher and send:-
"Great Uncle Bulgaria – the oldest and wisest of the Wimbledon Wombles and their leader."
But if you're spying on a pharmaceutical company, then your secret message might be:-
"Here we present a model of trabeculation in mice that integrates dynamic endocardial and myocardial cell behaviours and ECM remodelling, and reveal new epistatic relationships between the involved signalling pathways. "
There are not many books in the world that contain these exact words, and both message sender and recipient have had to possess one. So suddenly all books in the world are reduced to not many at all. A long bookshelf with 10,000 books is still only ~13 bits of book choice. And who has 10,000 different books incorporating the word "Wombles"? Searching through either the sender's or recipient's bookshelves would vastly impact security.
If Thomas Beale's house had been searched at the time, the treasure would have probably been easily located as he'd had to have a copy of the Declaration of Independence on his book shelf.
Comparatively, imagine if you could recover an AES key by using a side channel to reduce the unknown key space by 99%. That would mean looking for a better cipher. It's more secure and more practical to substitute letters rather than words. That way you can simply use the Bible for everything.