The best general introduction to acupuncture is Ted Kapchuck's The Web That Has No Weaver. Kapchuck's book presents acupuncture within the context of the Traditional Chinese Medicine (TCM) model. You should be able to find an inexpensive used copy on amazon.com. Mark Seem's Acupuncture Physical Medicine presents acupuncture within the context of a Western osteopathic model. This is, again, an inexpensive book, available through amazon.com. You may also find Tom Bisio's A Tooth from the Tiger's Mouth (cited in a thread that I introduced here a MT a couple of weeks ago) helpful insofar as it introduces concepts that, while not specifically related to acupuncture, provide an overview of Chinese medicine for the layperson. At around $11 from amazon.com, it's a good first investment - particularly if you are a martial artist interested in learning how Chinese medicine can help you manage your injuries. I found John Pirog's Meridian Style Acupuncture to be very helpful when I was a student in acupuncture school because it described a more classical approach than the two works cited above. At around $75 you may find this a bit steep if you are investigating acupuncture for the first time.
Best,
Steve