This was one of my textbooks as an undergrad (previous edition):
Physiology of Sport and Exercise 7th Edition With Web Study Guide
by W. Larry Kenney PhD, Jack H. Wilmore, PhD and David L. Costill, PhD
At that point, it depends on what you’re interested in. I assume endurance sports. The publisher Human Kinetics should have texts and books about specific topics. I would start there if you want to take a more academic approach.
I thought this lecture series was also good:
USA Cycling Coaches Clinic - Sport Science
https://podcasts.apple.com/us/podcast/usa-cycling-coaches-clinic-sport-science/id418387776
(one nice thing about listening to that series is that you’ll never be one of those annoying ppl looking for “scientific proof”. Lecturer does a fantastic job of elucidating how popular media, and society misunderstand how to apply scientific findings (at a high level).
Maybe start there.
If it’s cycling and power-training specific concepts that you want to understand, there is Coggan and Allen’s book, as well as the whole cottage industry that came out of that. (By using TrainerRoad you are a participant in that industry.) You could take a deep-dive into the WKO4/TrainingPeaks videos. You’ll learn a lot, but that stuff is very cycling (and power training) specific. The drawback there is that you might not be able to tease out the difference between basic sports physiology concepts (that apply across the board) and power-training jargon and ideas. For example, most physiologists in the world likely don’t know (or care) what FTP is, but they can tell you about lactate dynamics all day. The drawback to just taking the academic approach is that you’ll learn about something like glycolysis (for example), but not necessarily how to apply basic physiology to the real world (your day-to-day training).