Books about world music are at these call numbers:
Use advanced search in Library Search with terms such as names of countries, geographical regions, ethnic groups, instruments, and the term music or folk music.
General books on instruments are at ML460 in Reference and General Stacks. Dictionaries of instruments are in Reference at ML 102 .I5.
Some of the best sources in the library and on the Web are:
Musical Instruments of the World by 
A Survey of Musical Instruments by This page includes resources about the music of countries, regions, and peoples throughout the world.
Information and resources under the tabs above will also be useful, particularly: Dictionaries & Encyclopedias; Articles; Books; Audio & Video; and Interlibrary Loan.
The entire set of 10 print volumes is available here as a single, integrated online database. More than 9,000 pages of material and entries by more than 700 expert contributors from all over the world. Each volume contains an overview of the region, a survey of its musical heritage, traditions and themes, a description and images of specific musical genres, practices, performances, musicians, musical instruments, rituals, ceremonies, maps, and musical examples for further study.
The Rough Guide to World Music by Books on ethnomusicology are at ML 3797-3799. Here are two:
