These titles have recently been added to the Music Library Reference Room.  In no particular order:

Historical dictionary of the Broadway musical / William A. Everett, Paul R. Laird.

Bibliothecae apostolicae vaticanae corpus manuscriptorum musicalium / curantibus Rita Andolina, Susanna Greco. Vol. 2::1-4. Catalogo ragionato delle composizioni di Lorenzo Perosi (1872-1956) : con esempi musicali originali / Arturo Sacchetti. 

International who's who in classical music. 31st ed. (2015)

International who's who in popular music. 17th ed. (2015)

Klaviermusik : 55 Begriffe, die man kennen sollte / Annegret Huber.

A collector's journey : notable music books written prior to 1800 / Robert H. Cowden.

Die Bruckner-Bestände des Stiftes St. Florian : Katalog / Elisabeth Maier, Renate Grasberger. Teil 2: Gruppe 13-23.

Handbuch Dirigenten : 250 Porträts / Julian Caskel, Hartmut Hein (Hrsg.)

Sourcebook for research in music / Allen Scott (3rd ed.)

Sourcebook for wind band and instrumental music / Frank L. Battisti, William Berz, Russ Girsberger.

Dictionnaire des compositeurs francs-maçons : un lexique maçonnique / Paul Vandevijvere.

The complete book of 1970s Broadway musicals / Dan Dietz.

 I manoscritti liturgico-musicali di Bolzano : (secoli XIII-XIX) / Giulia Gabrielli.

Giovanni Battista Cirri : (1724-1808) : memoria e sintesi dell'inventio sonora / Elisabetta Righini.

Johann Franz Xaver Sterkel, 1750-1817 : thematisch-bibliographisches Werkverzeichnis / Joachim Fischer.

Eduardo Rodríguez-Losada Rebellón : catálogo de partituras / catalogación, Ma Dolores Liaño Pedreira.

Christoph Graupner : thematisches Verzeichnis der musikalischen Werke : Graupner-Werke-Verzeichnis : GWV / herausgegeben von Oswald Bill.