How can I improve my knowledge of pawn structure theory? What resources (books, videos, websites) are there to help me improve my understanding of pawn structures?
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 for the 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.

But as long as you don't mind skipping some sophisticated words like 'leucopenia' or 'melanoarchia', it remains an excellent ressource for studying pawn structures.