Okay, if you mean 'bk1' as in the very first book of a series, I read it like a doorway—one that creaks open and invites you into rules, faces, and a problem that won't let go. In my head 'bk1' sets the scene: the ordinary life of the protagonist, a disruption (sometimes a murder, sometimes a mysterious letter, sometimes a dragon landing on the mayor's roof), and the choices that push the hero out of comfort.
I tend to pay more attention to how the world is introduced. Is it slow, with little domestic moments that build empathy, or does it throw you into action and explain later? The best 'bk1's balance both: a hint of background sparkle (family, economy, grudges) and a clear inciting incident that lets you know what the stakes are. It usually ends on a promise—either a tentative victory or a larger threat peeking around the corner—and makes me want to pick up the next book. When I think of early examples, 'The Hobbit' or 'The Magicians' first volume vibes come to mind: introductions plus a map to the rest of the journey.