Here are some favorite books of Allen Simon, ChoralNet's Website VP:

 

Translations and Annotations of Choral Repertoire, vol 1: Sacred Latin texts (Ron Jeffers). If this book isn't on your desk, you're not a choral conductor. Every standard Latin text is translated both with poetic and word-by-word translations, along with historical and liturgical contexts, and lists of settings of those texts.

 

Anatomy of the Orchestra (Norman Del Mar). Everything (and I mean everything) that a conductor needs to know about orchestras: types of instruments, ranges, platform planning, handling players of different calibres, and more. Guaranteed to be well-thumbed after a few years.

 

Orchestral Music: A Handbook (David Daniels). This valuable reference work gives instrumentation and durations of thousands of orchestral works (not all choral), and has some glosses on different editions.

 

Group Vocal Techniques (Frauke Haasemann). From the late vocal coach of the Westminster Choir comes a detailed guide to teaching your choirs to sing correctly.

 

Singing Early Music: The Pronunciation of European languages in the Late Middle Ages and Renaissance (Timothy McGee et al.) -- this excellent resource discusses historical pronunciations of English, Spanish, German, French, and Italian (not to mention Catalan, Scots, and Portuguese), plus medieval Latin in various countries. A must for early-music lovers. You'll never perform Ceremony of Carols the same way again!

Harvard Dictionary of Music. Not really a dictionary; really a small encyclopedia, an excellent desk reference.

Bach's Continuo Group: Players and Practices in His Vocal Works (Laurence Dreyfus). Did Bach use harpsichord or organ in his cantatas? Or both? How often did he use bassoon for continuo? What about the violon, or the viola da gamba? Short or long accompaniment for secco recitatives? A fascinating analysis of the extant evidence from the manuscripts, with some surprising conclusions.