In the 20+ years since the Fallout franchise was introduced to the world, all of the games have been met with praise—which means that ranking them isn’t easy. It’s especially difficult to pit the games against each other when you consider that the post-nuclear franchise underwent a genre change.

For the past four years, writer Felipe Pepe has been compiling a book about computer role-playing games, enlisting developers and journalists from across the globe to write about everything from Ultima to Fallout 4. The book is out now—and you can get it for free.