The most important thing to realize about the NBME surgery shelf exam is that it's not at all about the technical details of surgery, but rather the medical management of patients who had or need to have a surgical procedure. It could be described as a medicine shelf with a surgical flavor. (If you've already done your Internal Medicine rotation, you're off to a great start.) To that end, the most important advice I can give is to know the purpose and indications of the surgery in question. For example, what is a laparoscopic cholecystectomy? When and why is it performed? You will not need to know much, if anything about the technical particulars, like location of incisions, suturing techniques, etc.
Without further ado, here's a list of all the resources that I used to prepare for the shelf, in no particular order.
UWorld: UWorld for Step 2 CK only contains 135 surgery questions as of this writing, but if you haven't already, get it anyway since it will be helpful for several other clerkships and of course, studying for Step 2 CK. The questions offer decent coverage of general surgery/abdominal stuff as well as trauma protocols - two areas that will certainly be on your actual shelf. Try to do at least 2-3 passes and make sure to hit all of your incorrects.
Dr. Pestana's Surgery Notes
If you only get one text for your surgery rotation, this is it. The venerable Pestana's book is essentially the Sparknotes of surgery - common surgical conditions distilled into one-paragraph summaries that include clinical presentation, diagnosis, labs and imaging, and management/treatment. Be sure to do the 180 questions in the back of the book!
Surgery: A Case Based Clinical Review (de Virgilio) Weighing in at 600+ pages, this is a thick text, but well worth it, in my opinion. It covers common surgical presentations in a case-based format split up by discipline (pediatric, lower GI, trauma, etc.). Each case comprehensively covers workup, relevant imaging, management, etc. If you don't have time to read it all, be sure to skim the 1-2 page summaries/cheat sheets at the end of each case. Finally, there are ~300 practice questions at the end of the book, which are well worth doing as a supplement to both UWorld and Pestana's.
NBME Surgery Self-Assessments
As of this writing, there are currently four practice exam forms available. Each one costs $20 and contains 50 questions. The price is obviously a bit steep, but if you feel like you need more practice questions or simply want an idea of how the shelf will be scored, it may be worth the investment to buy one or two. I certainly don't think there's much value to be gained by purchasing all four. Note that you will be able to see what you got wrong, but not the correct answers. However, a bit of Google-fu on the question stems will lead you to the latter.
There are many other resources out there (Case Files, NMS, Pretest, etc.), but as with everything in medicine, resource overload is a real thing. Stick with a few of the tried-and-true books and use them well. It's better to read through a few books multiple times than to read through many books once. This approach, combined with the resources listed above, should enable you to rock your surgery shelf.