[Utsuro no Hako]

I'll also add that,

Spoiler:

The first novel tends to gloss over stuff that happened between the first day and the 75th Floor -- there are epic events that could be the basis for a book on their own that get tossed off in a few lines (the crusade against the Laughing Coffins, the 25th and 50th Floor Boss Fights, etc.). Kawahara's been going back and filling in the details with side stories -- Aria explains exactly why the Beaters became hated, and Red Nosed Reindeer shows how Kirito's lying screwed the Black Cats. These should be animated in the next couple episodes.