Eric Walker at SF Site includes A Billion Days of Earth in his list of "Ten Overlooked Odd Speculative Fiction Classics":
Doris Piserchia urgently needs and much deserves a revival. In this dry, wry tale of the far future, humanity has become "the Gods," and the role of humans is played -- very well -- by evolved rats. When the mysterious and unstoppable Sheen arrives on Earth, only Rik cares enough to resist his dominion. Witty, amusing, thought-provoking, and ultimately quite moving -- this is a book to savor.