I'd have to sit down and count, but off the top of my head I would guess Stephen King. Mostly because they're good rereads, and with few exceptions I like them all.
11. Do you own multiple copies of any book? What are they? Why do you have multiple copies?
I do! I have a couple copies of Jonathan Strange and Mr. Norrell, the Mars trilogy, the Hyperion Cantos, and quite a few others. What generally happens is that I'll buy a book at a thrift store or Half-Price Books or the like, because I can't remember the last time I paid full price for a new book, and then later on will find the book in better quality, or hardcover instead of paperback, etc. At some point I'll be able to put all my books into bookcases and will probably get rid of the redundancies, but right now everything's in boxes for the most part and I don't worry about it too much.
12. Book borrowing – do you use the library? Do you prefer to try before you buy? What about lending your books to friends? Are you a good borrower, do you remember to return books?
Do I use the library? Oh, Lord, do I use the library. If I actually bought the amount of media I consume through the library - even at a way discounted price - I'd spend an absolute ton of money. I get books as well as movies and TV, and sometimes music, too. I've bought books from authors I like, or that I've heard good things about, before having read them, but never ever full price. Generally, if it's not less than a few dollars, I don't bother with it.
I was better at remembering to return my books on time when I still had to pay fines. :/
I don't generally lend books out? I just tell people to read books, but assume they can go to a library themselves. I have known some people who don't ever use the library, who only read books they buy at, like, actual bookstores, and I must admit I find this puzzling.