I carry Medical Student's Pocket Reference. It is a small purple book packed full of an enormous amount of info. It has to be about the highest information density of any book i've ever seen. It is basically a summary of 2 years of preclinical med school in a small 3 inch by 4 inch book. It has a few lines on just about anything you can think of in medicine.
I also use the palm 5MCC, which is pretty handy.
And, depending on the rotation, I keep the appropriate recall series book in my coat. I liked surgical recall and neurology recall. I think peds recall is pretty useless. I haven't done medicine recall yet, but it looks pretty handy (and thick).
i like to obsess about books!
-mrp