Some of the books I highly recommend:
12-Lead ECG: The Art of Interpretation - by far simply the best text I have found on 12-lead interpretation. This book is designed to take the paramedic from a basic knowledge all the way to advanced in an easy to read, fun format.
Guyton/Hall Textbook of Medical Physiology - indispensable for understanding the workings of the human body.
Prehospital Emergency Pharmacology - an invaluable resource for any and all drugs used in the prehospital setting, as well as in critical-care transport
Principles of Clinical Electrocardiography - simply the gold standard for serious 12-lead interpretation. A very advanced read that continually gives me headaches.
Critical Care Nursing - easy to read, comprehensive textbook for critical care paramedics and nurses.
Air & Surface Patient Transport - a great book geared towards critical care air and ground paramedics and nurses. An easy read with lots of valuable material.
The Human Body in Health and Disease - a great resource with basic anatomy and physiology, and mechanisms and pathology of disease in each body system.
I find myself in these books, as well as some others I will add at a later date, quite frequently. I believe that solid continued education and solid reference material is crucial for EMS.