if you're reading an old paperback book, how'd you make sure pages don't fall out or

You can try to very carefully glue it back it, or simply read the book and leave the page loose. There are paperback book archival bags made to protect the books if you want to go that route. I've wondered about -- but haven't tried -- microwaving a book with that problem to see if the microwave remelts the glue that holds the pages in. No idea if it would work.

I wont read it if it is very old and at risk of being damaged. I will see if I can find a digital version for my kindle.