Seeing this ad for the trade paperback edition of Dark Knight Returns from 1986 on the awesome Bully Says Comics Oughta Be Good! blog made me want to ask: What was the first graphic novel you remember buying? Note that by "graphic novel" I don't necessarily mean a self-contained work created specifically to be published in book form, but just comics bound up between soft or hard covers, as opposed to magazine form.
My guess is that it would probably be this one, published in the late '70s by Fireside Books in conjunction with Marvel.
It's a collection of key Fantastic Four stories from the '60s by Stan Lee and Jack Kirby, including the return of Sub-Mariner and the Galactus Trilogy. It was one of a series of books that included Origins of Marvel Comics, Son of Origins, Bring On The Bad Guys, and Marvel's Greatest Superhero Battles, among others. These were high quality trade paperbacks, aimed at bookstore chains and libraries, with lengthy introductions by The Man himself, and arguably marked the first time a comic company sought to curate its classic material in book form. But the format didn't really catch on, and it was another decade before the trades really caught on, thanks to Dark Knight and Watchmen. (I'm kind of leaving out the album style GNs of the era, like Marvel's God Loves, Man Kills with the X-Men, or DC's efforts in the same format like Star Raiders, since they were really more like big, high quality comic books, and you didn't really see that many of them outside of comics shops, anyway.)
Although come to think of it, I might have gotten the "Heavy Metal Illustrated Story" graphic novels of Alien and 1941 first, though since these are adaptations of existing media, they might not count... The 1941 adaptation by Rick Veitch and a pre-Swamp Thing Steve Bissette was really loose, though...