Any other cypher can be hacked by brute force.
Old fashioned book ciphers are incredibly difficult to crack especially when they use out of print books.
The way it works is I mail you say, five different out-of-print books. I number them 1 to 5. I have five identical books. Same edition. Same printing.
Then on my first message to you I select my words and identify them by (page)(line down/up on the page)(word left/right) taking care not to reuse any selected word from the book.
So my message to you would look like:
365-12-4 13-17-8 35-1-3 (and etc.)
You'd have the corresponding book to use to decrypt the message. It would be extremely unlikely that anyone else would have the same key.
You would write back to me using the next book and so on.