Rats in the Cellar II: Louis' Journey Home; Peter and the Lost Boys

In Rats In The Cellar, we met nine-year-old Oscar Wild Daugherty, who stumbled into a network of multiple universes, accessible via mysterious stairs at the top of his house and down in the cellar. He met various other instances of himself, and a cat, who is more than just a cat. This sequel picks up the story following the climax of Rats in the Cellar; this time, following Louis, the bad boy, in whom the cat takes a special interest.
After making sure Louis makes up for some of his nastier pranks, he sends him on a real adventure where he learns some fundamental facts of the multiverse. He teams up with yet another instance of Oscar, who takes the name Blake. They go on a rescue mission to find the owner of their house from yet another universe, who has become lost in the network of worlds. In doing so, Louis and Blake have found a home where they're accepted as twin sons of the man they rescued.
In Part 2, they're off again with their new dad, who has taken the name Peter to distinguish him from the other instances of himself. This time they deal with the various instances of "Uncle Milton", Oscar's pedophile uncle. There are more twists and turns as they meet another creature of the same order as the cat.