The Silmarillion, by J. R. R. Tolkien: It's beautifully written, it's filled with compelling tales, and it's the best book ever made. Period.
The Silmarillion is a book (or, more appropriately, a compilation of stories) written by Tolkien that covers the major events leading up to the third age of his world, ending at approximately the time the Lord of the Rings begins. It's one of the saddest books I've ever read: it makes me mourn for the beauty and beings that never were. One of the reasons I believe the Lord of the Rings stands above all fantasy (and other) books is that Tolkien took the time to create an entire history of his world, and by reading The Silmarillion you get a glimpse of that history. The Lord of the Rings takes on a whole new depth after reading The Silmarillion.
So why hasn't everyone read it? Well, it's long, it's filled with names, and it hasn't been turned into a series of movies yet (though it should be, darnit). Now, thanks to J. the Honourary Canadian, there's a solution to all that: The Entire Silmarillion of JRR Tolkien in One Thousand Words. It's a hilarious summary of the book: if you've read The Silmarillion you'll laugh hysterically, and if you haven't read The Silmarillion then you should go read The Silmarillion immediately (if only to appreciate the humor of J's summary).
A hearty thanks to Semantic Compositions for forwarding me the link.