Good for you for being proactive in your learning! As I'm sure you've heard, nursing school is stressful and you're smart for wanting to be ahead of the game. In addition to the suggestions already offered, I'd get familiar with acid-base imbalances (metabolic and respiratory alkalosis and acidosis) and know how to figure these conditions out by looking at lab values. Also, it's always good to know about normal reference values for different lab tests (platelets, RBCs, WBCs, electrolytes, and other things you might find in a CBC or CMP). I love this book and highly recommend it: http://www.amazon.com/Daviss-Compreh...7205611&sr=8-4
Not only does it have great explanations of blood/urine values, but it also does a great job of explaining other diagnostic tests. When I started out, I would have no idea what the nurses and docs were talking about when they threw around words like esophagogastroduodenoscopy, and I'd go home and teach myself from this book! Also, Taber's is indispensable: http://www.amazon.com/Tabers-Cyclope...7205781&sr=1-1
I wish you lots of luck in your studies!