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 art-book.
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.