I think that the easiest thing to do is to type in something like "20th Century books" or "best books" into a search engine such as Google and you will get a seemingly infinite number of lists, make your own selection from the ones given (or ignore them all if you feel that you know better).

As lists go, I liked this list of "important" books, broken down by year:
http://www.markkelly.com/books20th/

I think it would be interesting if we all listed our top five books and see where we end up.