Please buy the full DSM-5. Each diagnosis has good information on how to differentiate between disorders with similar symptoms.
Interview Guide for Evaluating DSM-5 Psychiatric Disorders and the Mental Status Examination
by Mark Zimmerman
This book has questions to ask patients to get the criteria for disorders. The pages on borderline personality disorder and bipolar disorder are excellent.
Medication Fact Book binder
by Daniel Carlat
The resident workroom has a copy of this book. It is much more efficient with less biased information than other books, notably Stephen Stahl.