Ooo..another one that can go in the FAQ list. I believe there's a good thread on this topic posted a couple months ago...
Basic of basics: Introduction to Clinical Radiation Oncology by Lawrence Coia. Small red book, easier to read, good start, but a little outdated.
The "mini" Perez: Radiation Oncology - Management Decisions edited by Cliff Chao. Stream-lined version of the Perez book. Hard to understand the rad onc portinos of it, but great for knowing basics (ie. nodal staging, draininage, etc).
Great general oncology textbook: Cancer Management: A Multidisciplinary Approach: Medical, Surgical and Radiation Oncology. You are not really expecting to know the particulars of rad onc so much as a MS, but you should the basics of the cancers, such as workup, general treatment, etc. This book is a good start for that.