Matthias Fritz & Brian Joseph 
Handbook of Comparative and Historical Indo-European Linguistics [PDF ebook] 
An International Handbook

This book presents the most comprehensive coverage of the field of Indo-European Linguistics in a century, focusing on the entire Indo-European family and treating each major branch and most minor languages. The collaborative work of 120 scholars from 22 countries,
Handbook of Comparative and Historical Indo-European Linguistics combines the exhaustive coverage of an encyclopedia with the in-depth treatment of individual monographic studies.