George Cain & Gunter H. Meyer 
Separation of Variables for Partial Differential Equations [PDF ebook] 
An Eigenfunction Approach

Separation of Variables for Partial Differential Equations: An Eigenfunction Approach includes many realistic applications beyond the usual model problems. The book concentrates on the method of separation of variables for partial differential equations, which remains an integral part of the training in applied mathematics. Beyond the usual model p