I stole all my D&D books, but I'm not evil. When I started playing I didn't buy the books. Then I decided to GM and I just borrowed a DMG and an MM from the old GM. Then I forgot who I borrowed the books from because they didn't write their name in, and they didn't ask for them back. Then I started writing in them, so when they do ask for them back I'm gonna have to buy a new copy.
I write in my books heavily. Whenever I look something up for the second time I recognize I'm going to need it again and enter its page number in the front of the book (eg, wealth by level, point buy character creation, wondrous items). Makes for much faster lookup.
I make notes in magic item tables about what the items do. Otherwise every single character you make, you have to look "What does boots of whatever do, what does orb of thunder do" while you're picking your items. You end up reading the whole magic item section every time. Better to note "stand from prone, daily" and be able to skim down the table looking for what you need.