WAIT HOLY SHIT THIS IS IT THIS IS PEAK FIC PROMPTING RIGHT HERE
Okay so headcanon that they find the journal in some hidden room that isn’t even on the floor plans of the house. It looks like it was put there to hide it, and they’re a little hesitant to move it, but they’re also too curious to leave it. They’re flipping through it for the first time in their room and Phineas is pulling a Dipper because this is so cool and he would kill to meet the author and all that, and Perry’s just on the end of their bed having a mental breakdown because oh shit this is it they’re gonna blow his cover and he’ll have to be relocated. There goes both his family and the entire Gravity Falls project he’s been working on all summer.
But while Phineas and Ferb do agree that they’re going to find the author and return his book somehow, their biggest takeaway is that they should make their own journals, too, and maybe leave those behind for someone else to find one day. They’re the ones to write the big 1 on the original journal, and then they start 2 and 3.
But Perry can’t just stop taking notes because his journal’s been found, so while the kids are sleeping, he grabs the journal and brings it back to his lil lair – the one he hangs out in when he’s off the clock, not his work lair. He leaves the boys out of it because he’s sure OWCA would not be happy if they heard that his host family had gotten ahold of his reports. He’d like to think that’s the end of it, but he knows the kids well enough that when they find it again, he’s not even surprised.
Phineas and Ferb are shocked to realize that the documents are still growing, even without an author in sight. They bring the journal back to their room, just to see if it happens again. This is probably when Candace finds them with it and demands to know wtf is going on, and Phineas is like “Oh, it’s a magical journal from a secret room! Ferb and I are gonna make our own journals – but we’re gonna have to write them; they won’t write themselves. Wanna make journal number 4?” and Candace just rolls her eyes and walks away, thus why there are only three journals.
And then when the journal moves again while they’re asleep, Phineas and Ferb try it a little bit differently. They stay up with the journal all night, waiting to see when it moves and when someone writes in it – and no one does. But then when they leave it unsupervised the next night, it’s back in the secret room but with two new entries; one new one for each day.
And now Phineas and Ferb are really interested in how this works, so they try writing to the author on the next blank page and leaving the book out for this mystery writer to take away again. Perry’s like oh shit because now OWCA’s definitely going to find out, but as long as they don’t suspect him, he can’t get in too much trouble, right? So he kinda invents the part of whatever the boys want the mystery author to be like. Every night, the boys ask him a question, and every night, Perry answers it from the POV of like a mysterious ghost writer or something idk and leaves it out in his lair for them to find the next morning. It’s the first time Perry’s ever really gotten to talk to them, and sure, he’s pretending to be someone else, but that doesn’t make it any less incredibly, both for him and for the kids.