Personally I am going to have to throw out Rangers Apprentice by John Flanagan. It's an easier read yet still pulls you in making it almost impossible to turn away. It's kept me interested in it even reading through the whole series three times (I have a lot of time on my hands most the time at work).
Another series would be Enders Game Ender's Game by Orson Scott Card. The Shadow series is my favorite of that whole saga. Last but not least is Assassin's Apprentice (The Farseer Trilogy) by Robin Hobb. It's a darker one yet I still enjoyed it.
Let's hear some of your favorites so I can figure out some more books to read on down time!.
If you like Sci-Fi, Dune by Frank Herbert is an exceptional read. The scale of the story expands from a single man to an entire universe over the course of the series. It explores things from personal, philosophical, religious, and political views. There are great, vicious worms over 400 meters in length, space travel that can only be achieved by beings living inside tanks full of narcotic gas known as Spice, and shape-shifting clones that can perfectly replicate another human. As the books go on, things get pretty weird, to be quite honest, but I personally found them to be very enjoyable.