Killing Floor is a great start! I started with this particular book myself and I haven’t regretted it. In it, you are immediately immersed in the atmosphere and understand who Jack Reacher is. And then you can move on down the list as you please. Personally, I liked Die Trying, which is the second book in the series. There’s a lot of suspense and a cool plot there too. As for the series itself, you can love it for the charismatic and independent hero who always seeks justice, even if you have to go against the system.