Ryan's Gambit (The Timeless Void Book 1)

Details: Jon Ryan had lived long enough to have learned his lesson. He has aided humanity on several occasions. He even brought Earth and all of its inhabitants back from the brink of extinction. Earth's timeline has been completely messed up as a result of his activities. Jon now feels compelled to restore the timeline's original sequence. If he doesn't, a slew of perplexing paradoxes will emerge.