How can I improve my knowledge of pawn structure theory?
The old classical book by Hans Kmoch, Pawn Power in Chess is almost entiely devoted to this subject.
It is often used as a reference because of its numerous useful examples, but also mocked but for author's tendency to over-theorize every principle and forge neologisms that have not made their way, half a century later, into common chess vocabulary.
Some excellent modern books on the subject are
- Soltis, Pawn Structure Chess
- Flores Rios, Chess Structures: A Grandmaster Guide
- Shankland, Small Steps to Giant Improvement: Master Pawn Play in Chess
There is still good content in the books of Kmoch and Nimzowitsch, though Kmoch's shortcomings are well enumerated in Evargalo's answer and Nimzowitsch tends to lay down general rules as if they are laws without mentioning how often they have exceptions (and chess theory has come a long way since his day).
"My System" and "My System In Practice" by Aron Nimzowitsch are classics with strong emphasis on pawn structures.