This is from the perspective of someone who did A residency, not an FM residency, but I think the same principles apply:
You're not going to review all the necessary knowledge of your residency or even that which you will learn in your internship in the few months before your residency starts. Nor do you need to. All residencies are on-the-job-training and are teaching-by-experience. You'll read to bolster your knowledge base, but reading some random article out of AFP won't stick with you as much as when you read that article when you're taking care of a patient with the disease in question. So going hardcore trying to read Cecil's, Gabe's and Nelson's before you start isn't likely to yield much. Read some fun stuff. For a medically related book read Groopman's "How Doctors Think"-something you should read at the beginning and the end of your training. I As for true true medical stuff, brush up on the basic concepts that may span disciplines but isn't overly broad. If you're weak with ventilator concepts, review that. FM residencies have a fair exposure to the MICU especially. If there are rotations that you haven't been exposed to in med school, maybe ask for some basic readings. A lot of FM residencies have you do a NICU rotation, but many haven't done that in med school. You could read about 5 or 6 topics and that actually might help you for that rotation. But again, don't wade deep into the esoterica. I would take a relaxed approach to the "pre-learning" though, because you'll be neck deep in the learning in residency. By now you should know how to write admit orders, H&Ps, progress notes, orders (esp. peds med orders); but if not, that would be worth brushing up on before residency.
