Reading an entire textbook you haven't been asked to is not an effective use of your time, especially if you're an M3 and preparing for Step 2CK because now the focus should be on clinical reasoning, not pathophysiology. If you're an M2, there's still a lot of pathophysiology to learn. Regardless, I would just find one comprehensive pathophysiology source, but rather than reading it, refer to it. This is what I would do:
1.) Create a OneNote on your desktop. Makes files based on field (OB/Med/Surgery) or subjects (Cardio/Renal/Heme)
2.) Find a comprehensive PDF. It can be your course syllabus, Pathoma, Harrison's, Robbin's, AMBOSS, etc.
3.) When you do UWorld and you run into an issue where you "don't understand the pathophys" ctrf+F the topic in the PDF and read that page or two. Crop that information and type a little in there.
I think it's more a matter of repetition, than a deficit that's going to be remedied by reading the best pathophysiology book cover to cover.