I would rather stick to the books and actually I may come up with some personal code. I need to think more about it.
Are you talking about writing in the pages of the book? I see a few book options:
1. Writing in the margins, or anywhere in the book, itself.
2. Use the existing letters in the book as some sort of steganographic code. There are many ways to do this.
3. Write on some other paper and glue it inside the pages of the book.
4. Carve out the inside pages, use the book as a container. Like how they put objects hidden inside books.
I am thinking you are leaning towards the second one. You can write a dot per page, above or below a letter. You would need 4 pages per word. 48 to 96 pages depending on number of seed words. Or spread it out even more.
If you have any sort of inheritance planning, I would not go this route unless you tell them how to decode your book code, or it's written down in the first page.
I think a lot of people should treat bitcoins (seed words) as some sort of precious object; maybe like a gold bar. Write it down. Seal it with white glue or whatever. Put it in an envelope. Store it in a safe.
A house fire can burn up to 1500 degrees Fahrenheit / 800 Celsius. That's why all these metal wallets have a market. I would just secure mine the way John Wick does for his guns and gold coins. I think those survive whatever fire razed his house.