How safe is a book cipher? How would you break it?
This isn't very secure. Generally, partial knowledge of the plaintext should not lead to leakage of other parts of the plaintext. In your book cipher it clearly does.

Say we guess the first part of the message. Then we can try and see which books would be correct for the given ciphertext. After the book (the key) is found we can then decrypt the rest of the message.