1. Hansen and Roach Handbook of Evidence Based Radiation Oncology. Divided by tumor site. Has pearls for each chapter, staging, recommended treatment by stage, and citations of trials backing up recommendations. New edi.tion any day now....
2. Haffty and Wilson. More of a textbook. I found the small type hard to take.
3. AJCC Staging manual, 7th ed. Has useful discussion of staging considerations and implications.
4. NCCN website. Free registration gets you access to treatment decision trees divided by site. Also has some multimedia presentations.
5. Hall's Radiobiology for the Radiologist - the first few chapters will help you understand the very basics of radiation interaction with matter, the linear-quadratic model of cell survivial, and units of dose and exposure.
6. The Hendee or Khan physics texts require little mathematical skill and would at least be useful as an intro to the machines.
7. Some sort of anatomy review, particularly of radiographic anatomy, would be great.
Most of these will be available in the department or your institution's library.