Not adventure related, but if you enjoy production artwork, I recommend the digital art-book Bioshock: Breaking the Mold
. Also, there's a really nice Half-Life series art-book titled Half-Life 2: Raising the Bar
, but to my knowledge there are no *ahem* legal
e-book versions for download.
Personally I'm crazy about these things - for a brief moment I considered buying the collector's edition of Alan Wake even though I don't own an Xbox 360 (nor was I very impressed by the story and gameplay after watching a playthrough), just to get the lovely looking Alan Wake: Illuminated
Edit: And if you're into LucasArts in general and not only the games, From Star Wars to Indiana Jones: The Best of the Lucasfilm Archives
is awesome - in fact I'm pretty sure I stole the art style for my game from the set design sketches for Indy 3.