Yeah, it's a bit of a tricky one because the names are the same, but they're completely separate works. Walter Kirn's novel 'Up in the Air' came out in 2001, and the 2009 George Clooney movie borrowed the title, the protagonist's name (Ryan Bingham), and the core idea of a man obsessed with airline miles. But the plots diverge pretty significantly. The book is darker, weirder, and more satirical about corporate culture and a mysterious company called MythTech. Ryan in the book is more of a cynical, passive observer caught in something he doesn't fully understand.
The film, which I actually love, streamlines everything into a more character-driven drama about isolation and human connection. It ditches the book's conspiracy-laden plot for a story about downsizing and a romantic subplot with Vera Farmiga's character. The movie's ending is also notably different in tone. So, it's an adaptation in the loosest sense—more of a 'inspired by' situation. If you go from the movie to the book expecting the same story, you'll be in for a surprise. The book feels almost like a different genre, a corporate thriller with existential dread, while the movie is a poignant dramedy.
I read the book after seeing the film, and it was fascinating to see the raw material they worked from. The film took the skeleton and built a different, arguably more accessible, body around it. Kirn's prose is sharp and funny, but the movie's emotional core is what really stuck with people.