The most challenging aspect of the Internal Medicine shelf is its breadth. From cardiology to nephrology to gastroenterology, the rule of thumb is that if there's a fellowship for it, there's almost certainly going to be a few shelf questions on it. Since most schools allocate 8-10 weeks for the Internal Medicine clerkship, chances are you'll only be exposed to a subset of all the Internal Medicine subspecialties. Thus, you'll have to do a decent amount of outside reading/studying to do well on the shelf, which can be difficult to do depending on how much of your day is taken up by clinicals. With that in mind, here's my approach to making the most of your time.
Even more so than perhaps any other rotation, UWorld for Step 2 CK is your best friend for Internal Medicine. The 1200+ IM questions - the vast majority of the question bank - provide a wealth of information that, in my opinion, sufficiently covers all aspects of IM, including biostatistics, medical ethics, and medical-legal reasoning. Start doing UWorld early on in your clerkship and aim for at least one 40-question set per day in timed tutor mode with all Medicine questions selected. Read and reread the answer explanations thoroughly and make sure to pay attention to tables and charts, as most of them are extremely useful. It will feel overwhelming and disorganized at first, and you'll probably do terrible at first, but over time your knowledge of IM will become more cohesive. In fact, it's totally possible to do well on the IM shelf using nothing but UWorld. That's how effective it is.
Step-Up to Medicine
If you feel the need to learn from a book in a more systematic and organized fashion, the perennial favorite is Step-Up to Medicine. Although the information is well-organized in a discipline-based format, I'm personally not the biggest fan of this book, as I found many of the disease presentations to be a bit vague and light on detail compared to UWorld.