From my experience, choosing a textbook definitely depends upon what/how you want to learn. Assuming you are looking for a decent "self-learning" text, I would suggest:
Medical Terminology Simplified: A Programmed Learning Approach by Body Systems by Regina M. Masters and Barbara A. Gylys
Medical Terminology Specialties: A Medical Specialties Approach
by Regina M. Masters, Barbara A. Gylys and Donald Venes
As the titles suggest, the first one might be a bit "too simplified" depending upon your background, although it does contain all the basics you'd need (i.e. how to translate terms, create combinations, etc.) The latter, however, is definitely a great source for learning important terms in various specialties in a much higher level. Consult Amazon.com or other reviews for further info.