books.google.com - This book provides much detail on the changes involving the grammaticalization of personal and relative pronouns, topicalized nominals, complementizers, adverbs, prepositions, modals, perception verbs, and aspectual markers. It accounts for these changes in terms of two structural economy principles....http://books.google.com/books/about/Grammaticalization_as_economy.html?id=e7yG6WSuwi8C&utm_source=gb-gplus-shareGrammaticalization as economy
