The books described here are meant to give you a high yield, high impact approach to each core clerkship you take. Ideally, for each clerkship, try to read one book throughly and use one book for case reviews / questions. Here is a break down of the books you should get, rotation by rotation:
Family Medicine is generally a nice rotation, with students primarily rotating in outpatient clinics. If you have already done pediatrics and internal medicine, family medicine covers many of the same topics, but in the outpatient setting. Preventative care is also much more emphasized. Blueprints Family Medicine does an excellent job of covering the major topics and preparing you for the shelf exam.
As discussed in the post on Books for the Internal Medicine Rotation, the three books above are all you need. Pocket Medicine will get you through the wards on a day-to-day basis while the other two are what you need to power through on your nights and weekends to ace the internal medicine shelf exam.
My recollection of OB/Gyn is somewhat fuzzy as I took it during fourth year just as interviews were starting up. The major challenge in OB/Gyn as I recall was learning the skills as well as knowing how to work up various conditions (such as an abnormal pap smear). For OB, just remember: almost always the treatment is - deliver the baby!
Hopefully the books listed above will prove as valuable to you as they have to me - best of luck out there in the wilds of the wards!