I've been using Pass CCRN. Link on Amazon
Amazon.com: Pass CCRN! (9780323025928): Robin Donohoe Dennison DNP RN CCNS CEN CNE: Books
Kinda mad about the price haha. I paid 80 and now they have it for 45 with free shipping. It is not only great for the exam (it comes with a CD with lots of practice questions that are great for the test) but it's a good reference book. It also covers all the critical care pharmacology you could ask for. They have a chart with pressors and what receptors they affect (alpha, beta, dopaminergic). I would say knowing all the different types of shock is a major thing for working in ICU too (hypovolemia, cardiogenic, neurogenic, and distributive (sepsis). Of course, it is all covered in this book. It's great too when you have a patient and go home and read that part of the book. I find that the information really sticks then because I can picture my patient!
I think it sounds like you are doing fine. Nursing is a life long learning process. I've told this to other new nurses on AN, but when I started in ICU, I would take home the report sheets (On my unit we have a little packet that covers basic info on every patient on our team including procedures, drips, etc) and I would highlight and look up drips, procedures, even conditions in their health history I was unfamiliar with so that I would be proficient the next time around. I hated not knowing something, or having a patient where I didn't understand what was happening. It sounds like you are motivated to do well. And because of that, you will!!!