If you find the PDF, you’re not just finding a book. You’re finding a ghost—a what-if scenario where the Master of SF handed over his sandbox to a stranger, and the stranger built a beautiful, terrifying castle.

While "2430 A.D." is not typically released as a standalone book, it is widely available in Asimov's short story collections:

He maintains a small, illegal collection of small animals (a shrew and some insects).