[gtpedrosa]

Tangent since it is not exclusive to image processing, but When I was studying Fourier Transforms at university I recall that the book "Fourier Series" by Gerogi P. Tolstov helped me a lot. Just throwing it out there in case it helps anyone, including to refresh the definitions the article starts with.

[capn_duck]

I read this book too and it really drove things home. I remember after feeling a little bit unsure about the relevance of orthogonal systems, feeling the thrill of beginning to see and anticipate where the argument was going. It felt like I was discovering it for myself. I think there are lot of good math and physics books from the Soviet era.