Lydia Auenhofer & Anna Giordano Bruno 
Topological Groups and the Pontryagin-van Kampen Duality [PDF ebook] 
An Introduction

This book provides an introduction to topological groups and the structure theory of locally compact abelian groups, with a special emphasis on Pontryagin-van Kampen duality, including a completely self-contained elementary proof of the duality theorem. Further related topics and applications are treated in separate chapters and in the appendix.