Introduction to Avestan: 1 (Brill Introductions to Indo-European Languages)

It contains a balance of linguistic and cultural elements of Avestan. There is an introductory description of the chronologic and geographic context of this language. The core of the book is a phonetical and morphological description of the language, with references to Sanskrit as well as to other PIE dialects and to PIE itself. There is chapter containing bilingual texts English & Avestan which I find very appropriate. A more advanced version of this book could include more about syntax as well as a more complete list of flexional paradigms.