Found my copy. I remember now, why I never wondered who Jack was, and why I was turned off by the book at first, and very nearly didn't buy it.

ZELAZNY'S EXCELLENT FORM...
It's not often you read a book that forces you to root for Jack the Ripper to save the universe, but Roger Zelazny is not your typical author... Macabre artist Gahan Wilson's drawings pack a potent punch." -- South Bend Tribune

Man, it's hard not to keep reading!

I almost didn't buy the book, and Zelazny's one of my favorite authors! Fortunately, I was able to get past it eventually.