Uptimers

A book about people who go back in time to change history until someone from their future tries to write them out of it. So how this time jumping works...you can go backwards in time from anywhere easily, set your co-ordinates and go. But to go forward you have to have someone from the future open a wormhole to you or you use a portable device knowing the date and place (4 pieces of information). When unknown uptimers cut off the one known future access point you have, how do you get uptime and stop them? And to compound the problem that someone in the future is sending more and more assassins back to try and stop you. That can only mean you are getting something right, right? Will you live long enough to find out? Lee Clyburn has lived a long life as an assassin and he's not about to let someone cut it short now.