I usually read simultaneously. I go back and forth between usually about 6 books at a time. However I was recently so hooked on a book that I read only it and finished it in 3 days. I hadn't been that into a book in a while.
I always read only one, and don't start another one until I have finished the current book... Though I have known others who read 3-4 books simultaneously depending on their mood...
Only one at a time. Usually. Sometimes if I'll read a little of Joyce Meyer or another Christian book. I don't mind "mixing" my everyday thriller book with an advice book since I won't mix the two up and get confused. I love Joyce. She's a preacher, and I really enjoy her advice books. I never mix anything else though. I usually stick with one at a time. Almost always a thriller though. Nothing quite does it for me like a good thriller. I have been nearly in withdrawals without a Mary Higgins Clark book to read the past month! I love her books. She's my fav author and I've ran out! Lol. I was supposed to make a trip to my bookstore today and didn't make it so I'm stuck reading my only 3 "back up" books. Lol. They just don't cut it like a good Mary Higgins Clark story though.
I only actively read one book at a time, but I'll often set one aside for awhile and read another. With nonfiction it's a lot more haphazard - usually I don't read any at all, but when I get interested in a subject suddenly I'll be poring over 10 of them at once.