I have done this exact job and know how overwhelming it can seem at first. The only reference material I could tell you to look at would be know your medical terminology, know labs (ask the lab for their book it explains what all the labs are and what they are for, I found this invaluable.) know radiology test, and get a drug book for the medications this will help you decipher some of your orders. Most of what you will learn will be from just doing it, but if you get to know some of the above things it will make your job a lot easier. Good luck.