Assuming that they're easily replaceable books from reputable sites, cull them. You'll probably find that you're interests change over time. It may be because something more interesting comes your way or it may be because you tire of a particular type of book. This means that you'll probably end up searching your library then the internet for something to read a couple of years down the road. That means that any time spent acquiring excessive numbers of books, managing excessive numbers of books, then searching through your library and not finding anything interesting will all be a waste of your time.

Of course, there's nothing wrong with keeping a few months supply at hand. You may also want to keep freebies that are still under copyright, because there is nothing in copyright law that says that authors have to offer those freebies indefinitely.