ADL uses a work concept developed together with the original service some 15 years ago by experienced reference librarians. It is completely pragmatic and related to what their needs were when helping patrons by providing pointers to texts in the information desk.
There the most common questions about literature were about
- individual poems, songs and psalms
- individual short stories
- novels and other longer pieces of prose (basically: "books")
- plays
With the exception for novels, most of these works are text fragments inside a volume, the unit delivered by the library's circulation desk.
For more information on markup used, have a look at the texts, and the markup used there.