For your browsing pleasure, these titles have recently joined our reference collection.  In no particular order:

A dictionary for the modern clarinetist / Jane Ellsworth

Béla Bartók : a research and information guide /  Elliott Antokoletz, Paolo Susanni (3rd ed.)

Thematisch-systematisches Verzeichnis der musikalischen Werke Engelbert Humperdincks / Hans-Josef Irmen (2nd ed.)

Encyclopedia of pop music aliases, 1950-2000 / Bob Leszczak

Die Musikalien im Diözesanarchiv Würzburg : Katalog der handschriftlichen und gedruckten Bestände (3 vols.)

The complete book of 1940s Broadway musicals / Dan Dietz

A dictionary for the modern trumpet player / Elisa Koehler

The Grove dictionary of musical instruments / edited by Laurence Libin (2nd edition)

Codici liturgici musicali del fondo manoscritti della Biblioteca Queriniana / a cura di Remo Lombardi