"I am now addicted to Adventure Time; actually, I am in awe of it." Director Guillermo del Toro expressed this opinion in his forward to this massive book of art from the cartoon series, and I agree with him.
I love everything about Adventure Time: the constrained-crazy character design, the candy-colored otherworldly sets, the untraditional-but-nevertheless-engrossing stories, and the bromantic relationship between Finn the Human and Jake the shape shifting dog, who embark on their Jodorowsky-esque adventures on a post-civilization Earth (all we really know is that our world ended as we knew it after the Great Mushroom War and the cartoon takes place about 1,000 years after).
This book is a deep dive into the Land of Ooo (the new name for Earth), with tons of concept sketches, marked-up scripts, and production art. It's also an oral history of the development of the series, with interviews from the show's creative team sprinkled throughout.
Adventure Time: The Art of Ooo, by Chris McDonnell