Nope, not to much. There are raws. Pretty easy to figure out the basics of how Seno knows what he knows.
I think its easy to fit a personality as detached as Hakushimas....all we know about him us that hes accepting but unapproachable....thats not hard to imitate
But if you notice, personality was exactly what it didn't fit. Natsuka didn't know Hakushima that well but several things 'Hakushima/Seno' did or said didn't match what he knew or expected... that's why he was surprised by many things. The book is a clue on how Seno knew something that could trick Natsuka into believing he was Hakushima and like he said, Nastuska is the type to believe what he wants to believe ... he wanted to believe Hakushima was willing to be with him and even if the personality didn't match he wanted to believe he had Hakushima. Seno personality though was what Natsuka got to know thinking it was the other man. Hakushima was more reserved, serious, cold ... Seno the other way around and he remained himself only using the other guy' name.
I think it's just creepy how he seemed to fit the other personality and everything lined up, but then it turns out it was just an act. But... how?? He has no recollection of the other party?... I'm thinking that the last panel, the one of the book, might be a clue. Who knows, maybe I'm just thinking into it a tad bit too much(?)