Book suggestions about eigenfunction expansion method in PDE solutions

Question

I want to solve non-homogenous PDE's (or PDE's system) by the method. I am looking for the notes, books etc. including theory and examples with solution about the method.

If I find right book, I can buy the book. But I don' t find adequate materials on the internet. (May be I don' t know right keywords for the search.)

Answer 1

in my courses I used the:

Applied Partial Differential Equations with Fourier Series and Boundary Value Problems: Pearson New International Edition

which covers basics like SoV, Greens functions, Fourier and yes, Eigenfunction expansions! It was a good introduction to the subject I'd say.

EDIT:

Also, google search for "eigenfunction expansion for pde" gave me lots of good hits.