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."
There is no one place to look to for online versions of books, because many books' full-text is restricted by copyright. However, searching in these resources will be an effective approach. If you have any questions, please ask a librarian.