At the risk of self promotion, my book (The Art of Agile Development) attempts to balance explaining the underlying theory with providing pragmatic "how to" guidance. A lot of the theory is set up in the first chapter, which is online here:

https://www.jamesshore.com/v2/books/aoad2/what_is_agile

The rest of the theory is spread throughout the book, in "key ideas" sidebars (which you can find in the index) and the "Experiments and Alternatives" sections at the end of each practice.

I'm not aware of any books that are purely about the underlying theory, but these books swing the pendulum more towards the theory side of things:

Lean Software Development (Mary and Tom Poppendieck) - if you only read one, this is the one to read

The Principles of Product Development Flow (Tom Reinertsen)

Agile Software Development, 2nd. ed. (Alistair Cockburn)

Extreme Programming Explained, 2nd ed. (Kent Beck)

The Lean Startup (Eric Ries)