The Cambridge Encyclopedia of the World's Ancient Languages: https://www.cambridge.org/ca/universitypress/subjects/languages-linguistics/english-language-and-li

The Cambridge Encyclopedia of the World's Ancient Languages: https://www.cambridge.org/ca/universitypress/subjects/languages-linguistics/english-language-and-linguistics-general-interest/cambridge-encyclopedia-worlds-ancient-languages?format=HB&isbn=9780521562560 https://www.cambridge.org/ca/universitypress/subjects/langua... (available elsewhere for less...) also has a good chapter on Old Persian. The chapter on Avestan is also relevant and interesting.