Andre Norton's books are good sci-fi / fantasy. You can get several for free from Project Gutenberg:
http://www.gutenberg.org/ebooks/author/7021 and many others from Baen.
Seconding Agatha Christie for mysteries, particularly her Miss Marple books. Also, Dorothy Sayers wrote good murder mysteries.
Timothy Zahn writes good action / adventure. His Star Wars series is probably best known and excellent. I also liked The Icarus Hunt.