G. H. Hardy & W. W. Rogosinski 
Fourier Series [EPUB ebook] 

Geared toward mathematicians already familiar with the elements of Lebesgue’s theory of integration, this classic, graduate-level text begins with a brief introduction to some generalities about trigonometrical series. Discussions of the Fourier series in Hilbert space lead to an examination of further properties of trigonometrical Fourier series, concluding with a detailed look at the applications of previously outlined theorems. Ideally suited for both individual and classroom study.