i thought Coulson had switched books since they were in a library of sorts.

I thought for sure that they were going to reveal that the guy only brought the bag out of the vault and left the book. But nope.

He was in a library. He couldn't have found a book the size and bulk of the Darkhold and switched them out in the bag?
This! A thousand, a million times this! The writers are really getting bloody lazy on the show, when they ignore such a blatantly obvious solution only to advance the story in the direction they want it. Coulson never used to be that stupid.
Probably the low point of an already bad episode.
"We're a team, Garrus. There's no Shepard without Vakarian."

Coulson obviously cared more about life of his Agent than a book of Evil
Funnily enough, I feel like Coulson is competent enough that he can care equally about both.
Giving the bad guys what they want to save an Agent is a very Coulson-ny thing for him to do: However, he has never actually given them what they wanted.
He gives them something else, something,
harmless.
Lincoln Campbell: I, uh, I never saw the original Terminator.
Phil Coulson: You're off the team.

Coulson obviously cared more about life of his Agent than a book of Evil.

Which just proves that Coulson was being a dum-dum again.
He was in a library. He couldn't have found a book the size and bulk of the Darkhold and switched them out in the bag?
He had to literally hand over the book (regardless of Daisy being there to take it away) and risk them actually getting it!?

I started to think that, too, but they did show the book with Radcliffe on the sub at the end.