As I just finished the book (loved it) last week, I definitely agree it's much different so far.
In the book his name is George, not James. He travels back to 1958, not 1960. He initially goes to try to save Harry's family, which is another town in Maine, not Kentucky. He only partially succeeds, so he returns to 2011 then goes back again. He then moves to Florida for a while before ever setting foot in Dallas. In the show, he went directly to Dallas, never saw Florida (so far, at least), and didn't go to try to save Harry's family until the end. I agree this will likely form the plot of episode 2. He also never met Sadie in Dallas, like he did in the show, only meeting her after working for a while as a teacher in a small town outside of Dallas. Also, the entire scene where he followed George to a JFK rally never happened at all in the book.
It's alot of changes right away, but I'm willing to be patient. It's a long book, even at 8 episodes they have to cut out some stuff to make it fit. I imagine they changed the order of events (going straight to Dallas rather than to save Harry's family) to keep the viewers attention. It takes several hundred pages in the book to ever get to anything related to Lee Harvey Oswold. TV viewers have a much shorter attention span.