There are several Subject Headings that will retrieve books on various aspects of musical theatre. Here are a few:
Choose Basic Search in the catalog and select Subject Heading from the dropdown menu. Then type your subject heading in the search box. In many cases, doing this will retrieve a long list of additional subject headings. Look through the list to see which subject heading best describes the kind of book you are looking for.
The Music Library has many librettos of musicals. To see a list of them, search with the subject heading Musicals-Librettos.
If you are looking for a particular libretto, for example, The Fantasticks, go to the Libraries' home page and select Libraries Catalog, then Advanced Search. Type the title of the musical in the first box, "fantasticks," and, in the second box, type the word "libretto," then click Search.
The Music Library has vocal scores and vocal selections of many musicals. The general call number for vocal scores is M1503, and the call numbers for vocal selections are M1507 and M1508. Within these call numbers, scores are shelved alphabetically by composers' last names. You can also "browse" the library's vocal scores and selections when you search by Subject Heading. In the Libraries' Catalog, choose Basic Search. Type "musicals-vocal scores" into the search box, and select "Subject Heading" from the dropdown menu on the right. For vocal selections, use Advanced Search. In the first box, type "musicals," and in the second box type "vocal selections." Here is a link to the vocal selections:
The Singer's Musical Theatre Anthology is a multi-volume set that is a good source for vocal selections. Each volume has songs for a particular voice type. This series is on reserve in the Music Library and may be checked out for three days.