Valanice: Imagine if books were only available three years after their release!
Coelocanth: The majority of them are. Many books go out of print within two years of release. The exceptions are mainly bestsellers and bestselling authors.
Ok, I admit that I said that poorly. The books are still widely available even after they go out of print, because libraries and used book stores have copies, and you can still read them. However, technology is usually out of date within 3-5 years--there's a new OS or console, and suddenly it gets harder to play the old stuff. But I can pick up a book that's 100 years old and read it as efficiently as I can pick up one released yesterday (well, almost as efficiently--publishing and writing have changed a little in that time, and it's noticeable). It's the retro-compatibility that makes GOG's work important--because I can buy a used copy of an old game fairly easily, but nothing says it's gonna work on any technology I have available at the moment.
(Although with ebooks, we might run into the same problems that games have with retro-compatibility. That could be as amusing as it is frustrating.)
Actually... comparing GOG's work to that of the scholars who dig through old books and produce easier-to-read and more-readily-available new editions is a very productive idea. I mean, as a literature student, I owe a lot of my time and work to those scholars who do that kind of archival work, the kind of work that allows us to read the classics without hunting down rare editions and then having to decipher old type-setting. And GOG's mission to make classic gaming more widely available is exactly equal (especially to scholars who are interested in vintage games, but also to the general media market). Unfortunately, GOG faces something that publishers working with 100+ year old texts don't have to worry about: copyright issues. And kudos to GOG for managing them as well as they have, even if it limits the available stuff.
So, yeah. Thank you for correcting me. I should have known better. And the correction has led to a great comparison.