Can I kill the caller Skyrim?

Hitting her with weapons will not affect her, but attacking her with Destruction magic will reduce her health. Once her health is reduced to zero, the system will treat her as dead and her body can then be searched. 360 Beheading the Caller as she teleports will cause her to reappear and be fighting while decapitated.

What happens if Orthorn lives?

If Orthorn survives Fellglow Keep while in the company of the Dragonborn, is not traded for the books, or if told to return to safety without accompanying them, he can be found at The Frozen Hearth in Winterhold.

Should I kill Orthorn?

Definitely not worth killing or sacrificing him. Unless you’re Urag. He’d probably kill him, resurrect him and then sacrifice him.

What does the caller drop in Skyrim?

As explained by Orthorn when you find him in the keep, The Caller is in possession of the three books that Orthorn stole from the College of Winterhold, namely Night of Tears, The Last King of the Ayleids, and Fragment: On Artaeum.