I agree with everyone else that your job as a medical student is to do a kick-ass job on H+P and then doing dictations. Find out who you are going to see the next day and do your reading ahead of time. It would be nice if you can read a Perez or Leibel chapter, but realistically it's very hard and too long for one night. When I was rotating, I used the Red Book by Coia, which is more concise. However, now that I know a bit more, I am realizing that it is very out-dated. Therefore, I actually would NOT recommend Coia. Same thing with Baby Perez, (the blue book edited by Chao) it's full of pearls and bullet points, but, in my opinion, not very helpful for beginners.
My recommendation is to read the NCCN clinical practice guideline, which can be found at
www.nccn.org. You can even log on and request a free CD to be sent home! It'll tell you how to stage a patient and how you would work-up, treat, and follow-up that patient; most practical questions you will be asked. It's concise enough for one night (at least the part that pertains to your patient). It is much more up-to-date than any textbook you will find. For natural history and epidemiology, though, you will have to go back to your favorite textbook.
One advice, when you are discussing with an attending (and when dictating), try quoting one study that provides evidence for your therapy approach. It's not that hard to find if you read the NCCN guideline (remember you don't really have to read the whole article to quote it), and, boy, it's very impressive.
In addition to doing H+P and dictating, some attendings may invite you to do the contouring. If they don't, I would be proactive and ask if you can participate in planning even if it's just watching over their shoulder. I didn't do this because, frankly, I didn't know that treatment planning was such a large part of what rad oncs do. Not only will it show your enthusiasm, it will make your day much more interesting.
Unless you are paired with an attending one-on-one, do NOT kill yourself trying to see many patients. Instead, see one, maybe two, patient a day and do a good job. Attendings don't know you are working your butt off to see four patients a day, they just remember you didn't do such a good job on that one patient of his/hers. If you have some free time on your hand, DO NOT sit and read; reading is for when you get home. The last thing you want to do is to look bored. Instead, I recommend spending time with non-physicians such as physicists, rad biologists, dosimetrists and therapists to get more rounded exposure to the field. Advertise you are doing this ("I want to spend some time today with therapists to see how they set up patients."). I believe this actually reflects positively... unless you use it everyday to slack off.
Lastly, for the most part, you will be doing just H+P and dictation day-in-and-day-out. There is much more to rad onc than that! Don't be turned off by your rotation experience and think this is a boring field. Unless you really need adrenaline rush to get you going, this really is a fantastically interesting specialty.
Good luck.