I recently picked up this lovely old book for a $1. It's not falling apart but it does have some loose places like this:
The threads will eventually break and the pages will start falling out. So I repaired it to stop that from happening.
I used a neutral pH pva glue that I bought several years ago for 50¢ at a thrift store and some cut pieces of acid free paper I bought for 99¢ for a pack of 50 at a dollar store several years ago as well.
This method is not for valuable books, those should go to a professional book repair person.
I ran a small bead of glue on each side of the crack, on the page itself, and used my finger to spread it thinly. Then applied one of the strips with the ends hanging over. I had used my thumb to run down the strip first, making it curve and then gently fold lengthwise. This glue is reversible with water and I used a lightly damp rag to clean up any glue residue and carefully shut the book for about 10 minutes.
Then I opened the book to make sure it wasn't sticking the pages together, trimmed off the repair ends, closed it and let it set up before moving on to the next crack.
The inside of the covers got the same treatment.
At this point all this cost me was time as I had all the items I needed.
If I could have found it, I have some white rice paper also picked up at a thrift store for about 50¢ and it's very thin but really strong. But it's packed away somewhere with other crafting supplies since my move.
You could easily use acid free scrapbooking paper, especially for endpapers. You would need to thin down the glue and brush it on to do large areas. I have used regular all purpose glue on newer books that have no real value but I prefer using the neutral pH glue on older books.
This cheap repair will keep this book readable for many years to come without worrying that it will fall apart.