I did repair a few comics in poor condition with tape; there was no way they could be downgraded further anyway, and it kept the pages together.

I used an eraser once on a comic marred by a greasy pencil. I also restapled a comic or two.

Other than that, I don't do much repair.

I will always try to flatten a creased book. Little risk of further damaging the book by placing it under a couple of heavy volumes.

When I have a book that is falling apart, I will sometimes use tape or a stapler if it means I'll be able to read and enjoy the book more easily. A book in that kind of condition isn't fetching any kind of resale value anyway.

Beyond that, no. No idea how to restore a book, and if I knew, I'd consider it dishonest. I don't collect for condition, so as long as the book holds together okay, I don't feel the need to do anything to it.

Cannot say that I have tried to "fix" a book, other than erasing pencil on a back cover once. I fear I would ruin it more or degrade it more so I leave my books as is. I do wish I had the skill to replace staples though...I have a few books where the staples are rusting in spots and I know it is only a matter of time before that bleeds onto the pages. I am not comfortable trying to replace them...I guess I could practice on some beater books I own.

I've circumcised some comics

I don't know the term for this, but once in a blue moon you might buy a new comic that wasn't cut correctly and one page at the corner is folded over and when you straighten it out you find that particular page is not a rectangle and the corner paper juts out from the rest. So you can either fold it back or cut the foreskin

I make a big production of this procedure. The trick is to do it ASAP so as not to cause the comic needless pain. There should be a Comic Book Mohel in the neighborhood, familiar with this operation. He'll come by with his bag of precision scissors. Invite the neighbors into your parlor, a little of the snip-snip, and then show off that new addition to the collection as youall partake of coffee, wine and bowls of pudding

You might want to save that paper foreskin in a baggie for when that comic gets old and might need  a paper-transplant.

Not me. In fact, as a youth I "ruined" many of them by drawing stuff on them. Yes, I'm a cad.

I've removed price tags from books bought at places like Half-Price Books, where they insist on using the old-style price guns to put stickers on their $.50 books.

I've also bought special erasers to take pencil markings and other smudges off of books, but other than these two acts, I pretty much leave the books as is.