"How many native-summoning Ardas are there with published works at least six hundred years in the future -"

214.

"So no books titled 'the defeat of Melkor', are there books about it -"

...here's a book...

He is pretty sure this isn't healthy or something but he'll read the book.

In this Arda the Valar sat around for ages and then simultaneously holed Earth and Endorë!

He shows it to Cam and might be giggling a little hysterically or something.

Cam doesn't do hysterical laughter but he looks at the page, sets his jaw, nods.

That is enough reading about other Ardas for the day.

Agreed.

That is enough reading about other Ardas for several weeks, but he spends them productively. Plays a game of Governor with Midnight and tells him about Matt and why it's probably not a good idea for the two of them to interact. Gets more catalogues and makes a very neatly organized list of things to offer daeva. 

Findekáno finds his apparent propensity for suicidal rescue missions amusing. But Findekáno is a hero in every single Arda they've read about, so. 

And at some point he is sitting in the Bar, reading (not histories of Arda, what would that help. He asked the Bar when no one was around if Fingon'd ever written anything, and he hadn't, and so he's been reading miscellaneous histories of non-Ardas) and another him walks in. "Hi," he says.

"Oh, I'm not Circle. If you were looking for him." The person he's addressing is also obviously not Circle, and not just because of the absence of Cam; this one is in a great deal of pain and basically indifferent to everything around him and 'indifferent' isn't a look that he's seen on Circle. 

The evil one. He looks tense. Probably because everyone hates him; that'd make it very unpleasant to come down here. He does it anyway, because loneliness is even more unpleasant. Matt's reflexive reaction is sympathy, he's so good at lonely people and here is a problem he can fix and one he vividly remembers. But this one is apparently lonely for a reason -

He defaults to neutrality. "Wasn't looking for anyone. I have instructions to stop you from opening the door, though."

So he knows that everyone hates Midnight, but not why. Probably Circle would have wanted to warn him without telling the whole story, Circle would have noticed immediately how fragile this one is... "I won't open the door. I won't leave just because Circle thinks we shouldn't talk, though."

"What exactly did you do?"

"Lot of things. My world's not as nice as yours. People use mind-control a lot more, people use binding oaths a lot more, there were political assassinations. I did all of that."

"I killed several hundred thousand people."

"I think they're giving you a pass because you're so miserable over it."

"I don't think so." They're - giving him a pass is the wrong word, Circle's considering it a close brush with fate himself, Edie and Emily have noticed how strongly their alts identify with his family and aren't going to try to persuade them otherwise, Cam's - Cam's sad. Cam's glad Circle won't end up in that situation. Cam's a mystery beyond that. 

"Are you going to fall apart, Circle was under the impression you are constantly inches from falling apart."

"I'm pretty sure if you hurt me very badly you won't be able to tell. I'd be even more sure, if it were anyone else."

"I had Nolofinwë assassinated and afterwards Findekáno wouldn't forgive me but I kept him anyway. People - sort of knew. Three hundred years. ...he's okay."

"No, he isn't."

"Have you considered asking him -"