I used Silverthorn's Human Physiology in undergrad and now in MS1 (and into MS2) we use Medical Physiology by Guyton and Hall. If you want to evaluate books before buying them, check out pdfdrive.com. You can't always get the most recent edition for copyright reasons, but you can get most books in an older edition sufficient to compare and see what you prefer. Medical school is fast and furious, so if you haven't taken human phys before, its a good foundational class. If you have taken some phys class, move on to anatomy. The other undergrad course that is paying dividends now is Histology, which you can teach yourself while the pace is slow using a book and online microscope slides and online self-tests. If you could list what you have taken already and what you plan to take before school, we may be able to give you a better-focused target of subjects to cover. At my school, the first semester is spent getting students up to par on a broad range of topics before we plunge next semester into organ based systems, and it included biochemistry and cell biology, anatomy, genetics and histology, then pathology ad immunology, and now we are working on medical microbiology. Students who have taken all of these as biology majors have a significant advantage over those who haven't. I had taken all of these except pathology and anatomy, and those are the two that I struggled the most with. My school's policy is if you fail any 1 of those 5 fundamentals classes, you can retake it over Christmas break, but if you fail 2 or more, you repeat MS1 entirely.