Like someone said before, as a med student, the MOST important thing you need to know is ANATOMY. I'll recommend several books:
1) Physical Exam of the Spine and Extremties: by Stnaley Hoppenfeld
Yes, the book was published in the 1970's, and its missing some newer ortho physical exam moves...BUT, the anatomy is unreal. If you know this book, you are 200 times ahead of everyone else that rotates with you. I cannot say how good this book is. Buy it....Hoppenfeld's books are staples of Orthopaedic education. Also HIGHLY recommended is his book Orthopaedic Neurology: a Diagnostic Guide to Levels.
2) I know some will recommend koval's handbook of fractures, but I think there is a better handbook: Handbook of Fractures by Clayton Perry.
Good, quick reading on classification, treatment, and easier to read than Koval's book. I believe this is THE idea book for intern's/2's and rotators, as well as anyone wanting to read a little before rotating. Good, practical advice!
3) Fundamentals of Orthopaedics by Miller. This is the baby miller, a good introduction into ortho, a good toilet book to have. Like the big Miller's Review of Orthopaedics, it is also in outline form, and sometimes reading it is not easy until you know a little about ortho. Good intro basic science chapters, etc...
4) Grab "Surgical Exposures in Orthopaedics" by Stanley Hoppenfeld before you start your rotations, or see if the librarby at your med school has one you can check out. Awesome for relevant anatomy.
When you are in the OR, you are fair game for anatomy pimp questions, and its much different from the way you learned anatomy in Med school, you don't see everything, only what's visible in the approach.
In all, I dont think there is 1 great single book. Fundamentals may be a good introduction....and for fracture care and management the Handbook of Fractures is pretty good....