I liked the first movie enough to start on the
book series. I've read about 8 so far (there are 20 in total) and really like them. It's a neat character because he's old-fashioned (typically doesn't use cell phones or high tech equipment) and wanders around the country, finding himself in new places where no one knows him and good and bad guys are always underestimating him.
If you liked the first movie, I recommend picking up Killing Floor (by Lee Child), the first book in the series. If you're like me, you'll enjoy it enough to want to read the next book, Die Trying (which might be my favorite), and so on.
BTW, the first movie was based on One Shot, the 9th book in the series. I suppose that you could start with that one, instead. You don't really need to read the books in order... even though I've more or less done that, anyways. I haven't read Never Go Back yet, since that's a more recent book, but I've been planning to read that before seeing this sequel movie, so I should probably start on that soon.
Not enough action for an action movie and not nearly enough mystery and suspense for a detective movie.
The series isn't trying to be either of those things, but a balance of a little of both. In that way, it's a bit like MacGyver, which might be why I like it. If you want pure action or mystery, MacGyver would disappoint, but if you're after a little bit of action with your mystery or a little bit of mystery with your action, then it might be right up your alley. Once you understand that it's not an action rollercoaster and it's not a Columbo, but a blend of the two, the Jack Reacher series is easier to enjoy, I think.