Jack Knife by Virginia Baker ISBN 13: 9780515142525

For Inspector Jonas Robb, each night brings new terror. But this night is different. It's brought two strangers-David and Sara-who have arrived in London seemingly with no past. What they do have is incredible knowledge about the Whitechapel fiend known as Jack the Ripper. Because David and Sara do have a past. It just happens to be in the future.Sent back in time, they're in pursuit of a 21st-century madman whose purpose is to change history. As the body count rises, Sara and David realize that their quarry and Robb's have become linked in a way that threatens not only Victorian London, but the very fabric of time.