I would like to ask for some books on Fourier analysis treating it in more abstract way (possibly more on orthogonal systems, not just focusing on the trigonometric fourier series), not the applications.
Edit: I had measure theory course, but still didn't listen anything on functional analysis or differential equations. My knowledge of physics is basic-leveled.
I think the book "Fourier Analysis and Its Applications" by Gerald B Folland has a quite abstract approach, despite its take on applications.
http://www.amazon.com/Fourier-Analysis-Applications-Applied-Undergraduate/dp/0821847902