here's what upper classmen at my school told me.
maxwell's to help you write progress notes at first and fill out charts. pharmacopia for generic drug names, pocket medicine (now in a red binder), pocket Harrison's/Washington Manual/Pocket Medicine for internal med, Sanford Guide to Antimicrobials (useful but not necessary), a Surgical Recall for surgery, Harriet Lane for peds, Tarascon for critical care.