partial differential equations – How to begin learning the eigenvalue and eigenfunction of Laplace

I feel the eigenvalue of Laplace is interesting. I want to learn and study it in the next two or three years. But I don’t know it. I just know some basic knowledge, for example, Moser iteration, Harnack inequation and so on. In my knowledge, there is not eigenvalue of Laplace. So, I want to know how to begin learning it? Namely, what book I should to read ?

PS: I just want to know the problem on $R^n$ not manifold.