Using phrases from books as your seed phrase seems obviously insecure. Even if you use a further encryption method of your own design.

What about passphrases, though? Let's say you have your seed properly generated & stored in a secure manner. How much additional security would a six word phrase from a book provide? What about six words, picked from one book, using a method of your own design? How would the entropy generated this way compare with the entropy of a six randomly generated words (such as throwing dice & picking words from the diceware list)?

Random six word passphrases are generally recommended as secure enough. Their downside is that while they are much easier to remember than secure strings of alphanumberic + symbols, they are still hard to remember, especially if you use more than a few. When using a method to generate 6 words from books, you just have to remember the method and which books you used. Is it comparably secure to storing your passphrases in a password manager or on paper in a hidden location?

In this way your wallet has 3 layers of protection: your seed phrase, your passphrase generation method and the knowledge of which particular print of a book to use. At this point I believe your biggest risk of loss is clearly yourself & the multiple points of failure you have set up