What makes them "essential geek books"?
Earthsea? It's considered a fantasy masterpiece, and it represents a different part of the genre than Tolkien. Also, it's written by a lady.
Delany... well, he's a personal favorite of mine. Delany is one of SF's best prose stylists, up there w/Vance (hmmm, another glaring omission). He's a great example of New Wave science fiction, in all it's ambition and willingness to move in new directions.
Nova prefigures both cyberpunk and all the smart new space opera that would come 20-25 years later, and
Dhalgren in sci-fi's
Ulysses (or maybe
Finnegan's Wake. Also, he's a gay black man.
The Parable of the Sower is wonderful, and it's social science fiction, which is underrepresented in the list, despite being a prevalent mode in SF writing. And Octavia Butler rocks, not only a SF Grandmaster, but a MacArthur grant recipient. As much as I liked
Snow Crash, to include that and nothing from Bulter sells geekdom way short...
Also, she was a gay black lady.
(note I'm not indulging in affirmative action for sci-fi... I picked these books and authors because I think they're seminal... however they also demonstrate the much more universal allure of geek-lit, for both writers and readers, which isn't relfected in that list)