A good introductory book would be Introduction to Clinical Radiation Oncology by Lawrence Coia, a small red book. It's a little outdated, but a good, simple, easy to read intro to rad onc. For a more detailed book, Radiation Oncology: Management Decisions by Clifford Chao (dubbed the mini-Perez book) is widely used by residents, but may be overkill for a med student. A good general oncology book that I like is Clinical Oncology by Philip Rubin.
The most important thing for a rad onc elective is not so much to know everything there is to know about rad onc and its treatments. Rather know the basics of oncology: clinical presentation, etiology, lymphatic drainage, general management, etc. Also, presenting something like a case or article for a noon conference or something is a good idea. If they don't require you to do it, then ask if you can.
Another great "intro" book is the World health Organization's Radiotherapy in Cancer Management: A Practical Manual
by J. Stjernsward and G. P. Hanson. This actually is less valuable than Coia as far as the fundamental theoretical underpinnings of rad onc, but is superior in terms of clinical utility (i.e. setting up fields). I found it a little outdated (no IMRT) but better for clinical rad onc than anything else I've seen.