Since most of his memories involve Ran in some form or another he'll definitely remember her from the start, buuut the whole 'oh i'm also in love with her' thing might not come til a little later (but probably within his first few memories, really. BOY IS WHIPPED). So if he finds out she's in the book before that, then he'll sadface a bit but mostly in the sense that gosh if only she weren't stuck in there I could glean more info about tiny me from her :|!!!

If he DOES properly remember his teenage boy feelings for her before that, then he'll be extra sadface and possibly interrogate some Personae on the chances of getting someone out of the book. (and possibly do something appropriately dramatic but unnecessary like ask to see the book personally then go 'Wait for me, Ran' or something while no one is looking. BUT DETAILS.)

I know Kit will outwardly pokerface and be a troll to him when he arrives, but how will Kit internally react to Shinichi showing up?