Understanding Patient Safety, 2nd edition. It's a single authored text by Bob Wachter, one of the pioneers of the patient safety movement. I found it to be a top-to-bottom overview of the topic, filled with useful examples, and not only easily readable but also an enjoyable read.
This is a useful book for just about everyone in healthcare, and probably should be required reading for all health sciences students. I think it's of particular value to hospital epidemiologists because many of us may not be aware of developments in other safety issues outside of HAIs.
You can view the table of contents here.