Here are a few good sources that I have used over the years:
Legged Robots that Balance by Marc Raibert. Originally published in 1986, it is a little hard to find a copy of this book, but if you can it is a great read. Raibert's work on legged locomotion was the impetus for much of the progress happening on legged locomotion. Today we are still trying to understand formally some of the concepts that he showed empirically.
Edx, and Coursera course from UPenn (https://www.coursera.org/learn/robotics-mobility?specialization=robotics, https://www.edx.org/course/robotics-locomotion-engineering-pennx-robo4x). These two online courses do a good job of explaining many of the core ideas of legged locomotion. The coursera course is more conceptual and the edx course is more mathematical. I would recommend doing the edx course if you can, although the content is more difficult. There is overlap between these courses and Raibert's book as the courses try to show why some of Raibert's robots worked.
Russ Tedrake's Underactuated Robotics Course on MITOCW. (https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-832-underactuated-robotics-spring-2009/) I have not gone through this course but I have heard great things about. Goes through some of the post modern (optimization) techniques used in legged locomotion.