Reddit mentions of ACSM's Introduction to Exercise Science

The first overall source I'd look to for Ex-Sci is a textbook from Mcardle Katch & Katch it's a bit more user friendly for getting into the field.

Another good source for info is the American College of Sports Medicine (ACSM), and they have an Intro to Exercise Science as well. They're a bit more Science and Research Heavy, so they can be good or bad depending on the reader.

To get a good starter for musculature a very helpful one is Strength Training Anatomy This one is only a very colorful and visual source of where the different muscles are and how they're involved with different movements.

Supertraining was mentioned earlier in the thread, and is an Amazing source for how different training variables and methods affect the body.

I've found Exercise Metabolism very helpful in how the body uses different macro-nutrients in various intensities of physical activity.

One of my favorite books is also the Essentials of Strength and Conditioning from the National Strength and Conditioning Association (NSCA). It's more geared toward programming for athletic pursuits rather than overall physical fitness, but it still does give a great understanding of training variables and the body's adaptations to them.

EDIT: The subject of Kinesiology is touched on in most resources, but you may also want to get a standalone resource for this if you want to really understand the construction and functionality of the musculoskeletal system. The courses I've taken and research I've done have used a lot of different resources, so I don't have a single one personally to include here.