Bybee Joan L. Bybee 
Morphology [PDF ebook] 
A Study of the Relation between Meaning and Form

This is a textbook right in the thick of current interest in morphology. It proposes principles to predict properties previously considered arbitrary and brings together the psychological and the diachronic to explain the recurrent properties of morphological systems in terms of the processes that create them. For the student, the clear discussion of morphology and morphophonemics and the rich variety of data brought in on the way to the theoretical conclusion is material for a direct learning experience.