Lange's Cardiovascular Physiology is wonderful. It's relatively concise (256 pages)and the book is physically small, making it seem even shorter. Better yet, it's understandable and the diagrams are actually helpful. It's just what you need. Our school made us buy this text because Berne and Levy's Physiology was so cryptic (I still get nightmares of that thing).
As mentioned before, BRS Physiology by Costanzo does a good overview. It covers the essentials and is handy before a test or shelf exam.
You probably don't need Dubin's or an EKG book right now, unless your school covers EKGs first year. I remember only one EKG on Step I, and First Aid covers what you need to know for those. I've had to learn EKGs for a clinical course and internal med, but I quickly forget what I learned since I didn't see them on other MS-III rotations.