When we moved from a three bedroom house to a duplex when I was a kid a lot of the ceiling to floor wall of books in the converted den had to go. Most had been acquired cheap and second hand (the old Army/Navy store was good for that) but all were quality of their type. The Book of Knowledge set was so old it was as much storybook as encyclopedia, and had quaint old timey cars racing around the sun to represent the planets--all eight of them. Those and a BUNCH of others went to the Lutheran parochial school I'd been attending, which had only recently scraped up enough money to even have a library BUILDING, let alone books.

Now, obviously, graduate level history texts aren't as well suited to a K-8 school as many of those books, but the same principle applies. I'm sure there's an urban APUSH class or something nearby that would think your collection worth its weight in the gold they don't have. Or a small community college without former governors and Fortune 500 CEOs among their alumni to make donations.

As to your questions, almost every single one of my books has huge sentimental value. Many are still among those my parents acquired in my childhoood. There are a few more recently acquired ones of which that isn't true; I suppose I'm a bit cold toward cheap and poorly written paperbacks, not above buying something that catches my eye, reading it that night and coming back with the receipt by the end of the week. But for the most part they're family, and you don't give away family (tempting as it might be now and then.... ) And, be honest: I wouldn't be nearly as much fun if I didn't own at least ONE Lee Strobel book.