My 2 cents worth: this is all from my personal experience of having too many books and no space and still buying more books, so needing more space.
Permanently get rid of the books you know you are never going to read (again). Let someone else who might appreciate them more, read them. If you are really not sure, store them and if you haven't looked at the books in there or even thought of them in about 3-5 years or so - get rid of them.
The reference and non-fiction - depends what it is. If it's outdated, get rid of it. If it's on a subject you no longer have an interest in - get rid of it. If it's a cook book you only use one recipe out of, photocopy the recipe and get rid of the book. If it's on a subject that will help you survive, food preparation/preservation from scratch, or fix thing when the society goes to hell and you no longer have reliable electricity and water etc then definitely keep that on premises (either on the shelf or in a box in the garage or some such). You can't look up stuff on the internet if the internet is gone or not available. Personally, I tend to keep the nonfiction (science & history) books and get rid of the fiction (unless the story is really good and I intend to re-read it at some stage).
There is also the option of storing books in double layers on the bookshelf (if your shelves are deep enough for this). For example, I would put David Eddings' Belgariad series at the back and his Elenium series in front of it. That way I know David Eddings books are in that particular spot on the shelf, even if I can't see all of them. I tend to shelve my books roughly by genre; so 1.5 cabinets fro history; 1.5 cabinets for science; 3/4 cabinets for the odd-ball/alternative knowledge type books; 1 cabinet and small book case for fiction, 1 cabinet for classics (which are a mix of fiction and non-fiction, like Caesar's commentaries, Herodotus' histories etc) - that way if I'm looking for a particular subject/genre I don't have to go through all 2000 books, just the relevant sections of shelving - this is especially useful if the books are double and triple packed. The fiction and odd-ball shelves are the only ones double packed these days - I got rid of approximately 70% of my fiction a few years ago (the boxes had been sitting in my parents garage, untouched, for about 10+ years, so time to get rid of them).
NB: If you are going to store books in boxes, number each box, make a list of the books that are in each box, and put a physical copy of that list in the box with the books, and also keep the other list more readily available, like on the computer or in a file. That way you don't have to go though half a dozen boxes of books just to find one book. You can just look at the list, and then haul out that particular box. UPDATE the lists if you add/remove things from the box!
I also recommend plastic as opposed to cardboard boxes for long term storage because they don't disintegrate or get the contents soggy when they get wet (invariably something leaks) or get eaten by creepy crawlies. I also recommend transparent plastic boxes as opposed to opaque plastic boxes so you can see at a glance that a particular box has books as opposed to fluffy toys, or craft project or tools or whatever - but this depends on how much "stuff" you have stored.
Sorry for the long diatribe, but you did ask.
