Rats in the Cellar III

The final part of the Rats In The Cellar trillogy: In the first part, 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. In the sequel, the cat takes special interest in Louis the "bad boy" of the lot.
After making sure he does good by people he's hurt, he helps him find Blake yet another instance of Oscar, who's been living on the street. Together, they rescue Jacob Ogbu, the man who owns their house in his universe, but has become lost in the multiverse. They have more adventures together, and the Ogbu family accept Louis and Blake into their family. In the course of their adventures, Blake finds an old diary...
In this sequel, the final part of the trilogy, Jacob Ogbu reads the diary, which tells the story of how the time travel experiment of a 21st century scientist backfired, and how he got stuck in the 18 century. Without intending to, he imparted the secrets of time travel to the wrong people, who had financial interests in the slave trade. With their knowledge of time travel they pre-empted the abolition movement as well as that to end child labour, thus dooming that universe into the darkest dystopia of all.
However there is hope...