Lakshmikantham 
Introduction to Nonlinear Boundary Value Problems [PDF ebook] 

A book on an advanced level that exposes the reader to the fascinating field of differential equations and provides a ready access to an up-to-date state of this art is of immense value. This book presents a variety of techniques that are employed in the theory of nonlinear boundary value problems. For example, the following are discussed:- methods that involve differential inequalities;- shooting and angular function techniques;- functional analytic approaches;- topological methods.