I've spent a lot of time there. Go to the Performing Arts Reading Room (formerly Music). Card catalogs start at back right as you enter. 1st-Author/Composer, 2nd-Title, 3rd-subject. What are you interested in? I might have some tips for searching (18th century music is wretchedly cataloged, and in the card files some cards are out of alphabetical order). 'A collection of old ballads' turns out to be a collection of single sheet songs with music. Two manuscript collections have catalog cards that are identical except for the # of pages in the manuscript. There are two sets of McGlashan's 3 books of Scots tunes. One set is missing 1 of the 3 title pages, and of the 2 tps remaining, 1 is bound in at the front of the wrong book. Lots of little things to learn.