Vit Bubenik & Silvia Luraghi 
The Bloomsbury Companion to Historical Linguistics [PDF ebook] 

Originally published as the Continuum Companion to Historical Linguistics, this book brings together a number of leading scholars who provide a combination of different approaches to current and new issues in historical linguistics, while supplying an exhaustive and up-to-date coverage of sub-fields traditionally regarded as central to historical linguistics research. The editors aim to build a solid background for further discussion and to indicate directions for new research on relevant open questions.

The book includes coverage of key terms, a list of resources, and sections on:
history of research
methodology
phonology
morphology
grammatical categories
syntax
grammaticalization
semantics
etymology
language contact
sociolinguistics
causes of language change

It is a complete resource for researchers working on historical linguistics.