Fair enough, but I wouldn't say it's exactly clear in-game. I find fit dubious he would write an entire book, just because the inquisitor says so. But that's just me
If you ever change your mind again, we have sugar and rainbows in the varric/cass thread 
Dorian: So Varric, are you and Cassandra... ?
Varric: What? No! Why would you even ask that?
Dorian: Truly? Bizarre.
Cassandra: I'm right here!
Dorian: See? She's right there. What are you waiting for?
Varric: Just because two people dislike each other doesn't mean they're about to kiss, Sparkler.
Dorian: Not according to your books.
Varric: Don't mistake me for that hack who wrote Hard in Hightown II. I can spell.
Seems pretty clear to me - Varric opposes the very idea Cass and he are in love. And for all his lies, I believe him. It's Bianca he's desperatly in love with. Not that last part doesn't annoy me (I had hopes for a varricmance back in the days) but, it is at it is. Him writing the book for Cass is just an offer of peace - not that he would ever admit it. Because yes, he did lie to her and to the Inquisition, and he's very lucky the consequences weren't worse.
So, yeah, that ship has sailed, and I wasn't on-board.