Even brillianter advice from your Auntie Kriz, Kathy: I've literally just disposed (two days ago) of two bin bags of assorted books between the Ancient Briton and myself to a book dealer. We got £40. Look up 'Book Dealers' (yeah, I know it's obvious!) in your Yellow Pages. Any of them saying they deal in antiquarian or second-hand will come round, VERY quickly whizz through what they want, and make you an offer. They'll cart them away and leave you with what you can then either flog off at a car boot sale, or donate.
What they'll want to find are good condition fiction and non-fiction paperbacks that aren't too old; all kinds of good hardbacks on non-fiction subjects; leather-bound or genuine antiquarian books (not just old Victorian novels, though!).
There are lots of subjects they won't be interested in, and they never buy Reader's Digest stuff (because far too many people do!), but let them sort that out.
The best thing is NOT to try and sort them yourself (I never do when I'm culling the shelves) - just leave them to take a pull at what they want. You'll be amazed at how quick they are! You won't get much for anything modern, to be honest, but if there are any books that look special to YOU, put them aside and ask the dealer about them. Sadly, unless it's something like a first edition of T S Eliot's 'Old Possum's Book of Practical Cats' (on which 'Cats' is based), or a really rare book, you won't get a fortune. But you might just as well have whatever they give you, and clear the space!