To be certain that you're looking at all of the books on a particular topic, it can be useful to search the official library subject. Sometimes the library subjects are not intuitive, sometimes they are. There may be more than one subject term that you'll need to look at for your topic -- both poetry and poets, perhaps. You can use either the basic or the advanced search options to search for subjects; in either case, use the pull-down menu to select "subject."
This page contains key resources to for Spanish, Portuguese & Catalan books online.
For theater-specific resources, please check the 'Theater' subject guide.
CUNY DSI has an online Spanish Paleography Digital Teaching & Learning Tool.