Because it was the book that opened my eyes to what could be done with story telling, with language, with erudition and wit. Because every time I read the book and its in-series sequels, they challenge my intellect, expand my vocabulary, make me think about both the largest questions and the smallest, and most of all, give me the chills. Because it's a damned good book.
Other stories/books/series which have set barbs deep in my reading mind:
"Story of Your Life"
City of Saints and Madmen
A Song of Fire and Ice
The Einstein Intersection
Fifth Head of Cerberus
Vellum and Ink
What's your favorite book (or series)? Why?