This was the first Jack Reacher book published.
However, it is not chronologically the first story in the life of Jack Reacher.
I suggest first reading these short stories in this order: Second Son, High Heat, Deep Down, Small Wars. They can be found in the 12 story collection titled "No Middle Name".
Then read: The Enemy, Night School, The Affair before reading Killing Floor.
Typical Reacher book; mayhem, murder, mix-ups, retribution, resolution, renewed roaming.
I did enjoy Lee Child's introduction which explains why and how he started writing the series.