Archive.org has many out-of-copyright books, but there is little support for discovery of "related books" or "all books in a multi-volume series". Sorting by download count within categories is a start, for example:
https://archive.org/search.php?query=mediatype%3A%22texts%22... will lead to The Cambridge History of ___ (geography or topic, e.g. Literature, India) and The Cambridge ___ History (time or topic, e.g. Ancient, Medieval, Natural). Each of these titles are several volumes, 500-1000 pages per volume, covering centuries of events from a British perspective.
https://archive.org/search.php?query=mediatype%3A%22texts%22... will lead to The Cambridge History of ___ (geography or topic, e.g. Literature, India) and The Cambridge ___ History (time or topic, e.g. Ancient, Medieval, Natural). Each of these titles are several volumes, 500-1000 pages per volume, covering centuries of events from a British perspective.
German Classics, https://archive.org/search.php?query=subject%3A%22German%20l...
Eastern Classics, https://archive.org/search.php?query=subject%3A%22Oriental%2...