Use Cecils questions or Harrison's questions . These are ultra-hard questions but the answers integrate a lot of knowledge. The answers are very helpful. The best overall NMS for a clerkship. Some people placed OB pearls in their pocket for a reference.
PreTest questions are good review for the test.
The Essentials of Ostetrics and Gynecology was recommended by many residents. It contains all the information that you might be expected to know and it is very easy to read. A good book. This area is controversial. NMS was used by many but almost everyone said that it was not a great book.