have an open secret: There are (mostly were) apartments above 30 of them.
When the library system was built early in the twentieth century, apartments were included on the top floors of the branches funded by Andrew Carnegie. These quarters were built so that custodians could live alongside the library. While living in a secret apartment above a library seems like a dream to most of us, it was a practical reality for almost a century for the NYPL. It wasn't particularly glamorous, though: The main reason for these round-the-clock custodians was that the branches funded by Carnegie were heated by coal furnaces, so the job entailed shoveling coal to keep the furnaces running.
Today, only 13 NYPL apartments remain. The rest have been re-purposed, as the need for coal-shoveling has been eliminated, and other demands on the space have arisen. Some of the remaining apartments have been deserted for decades, and Atlas Obscura got to take a peek inside.
For more images, check out this Atlas Obscura article.
You might also enjoy this Gothamist article, which includes this tidbit:
There were also two non-Carnegie branches (George Bruce Library and Ottendorfer Library) which had similar apartments. In 1949, an advertisement for a custodial job at one of the branches listed a monthly salary of $60.83, and with that salary came a five-room apartment (ft. a dumbwaiter!) in the building.