I bet the dream was a guilt trip, an alteration of his memories based on his experiences in his modern life. He stole the book just like in the dream; rather than being found out, however, he got away with it, therefore Flora never got back. But he didn't get rid of it, no. He's only now getting guilt dreams over it because of her reaction at the expo and his inability to remember the whole ordeal, and wants to get the book to perhaps obtain a clearer image of how the event unfolded. The reason he's discovered in the dream is his innate desire to have been caught at the time, combined with his idea that Creepy Cats get in the way of all his plans, thus giving him a catalyst for that event to unfold in such an altered way.
Which, if I'm right, also means we can probably cross out a couple of ridiculous events such as [Flora being the Creepy God] as Oscar's disposition towards creepy cats as omens of his own inevitable failure. My boy is starting to grow up.