Short answer: Use a bookmark, avoid folding pages, keep books dry and return them on time.
Handling
Avoid dog-earing pages and keep drinks away from books.
Storage
Store books upright in a dry place and away from direct sunlight.
Return on time
Return books promptly so the next reader gets the same quality.
Frequently asked questions
Can I write in a borrowed book?
Avoid it. Use sticky notes or a notebook.
What if a book gets damaged?
Tell the owner promptly and follow the agreed terms.