This may not be the top-selling item at Amazon, but it's an excellent book, put out by Brill Publishing (Leiden), who make great scholarly books. There's a Gotisches Etym. Wörterbuch out there by somebody, but this is much easier to use and more helpful (plus it's in English, which is more convenient for most of us).

What really makes this nice is that, not only does it give Indogermanic cognates for the Gothic words, but it tells you which Greek words the Gothic ones are translations of (from Bishop Wulfila's Bible, who studied in Constantinople). Also, the book is sturdy and has a good feel to it.

This book is unique in that it presents a huge mass of scholarly material and is extremely readable at the same time. The way this is accomplished is that the development of theories are addressed through the history of their development. It's amazing to see a topic like this comprehensively treated in an almost narrative manner. Aside from the enjoyability of the read, you get the benefit of learning two things at once: (1) The Theoretical Bases of Indo-European Linguistics, and (2) the history of the scholars and their work (or how (1) got that way). Essential for anyone interested in IE Linguistics or the history of scholarship (bonus if you're interested in both).

The author of this book, Prof. Winfred Lehmann describes very systematically on the theoretical bases of Indo-European linguistics, referring the existing standard handbooks with valuable criticism. He introduces the modern methodology and principles on the phonology, morphology and syntax for the Pre-Indo-European and Proto-Indo-European. This book is highly recommendable for the beginners and experts on the Indo-European linguistics.