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To be honest i was first introduced to it by reading "one shot" and it is a fairly recent one. there has only been "the hard way" and this new one since it.
i then read "the hard way" it was after these two that i was hooked and read them as i could get them. There was minor references to other storylines from other books (relationships etc) that may incline some people to want to read them in order, but it didn't bother me as the stories were that great. they were like fiction books with that element of fact within them if that makes any sense.
the only one that doesn't matter if it in order or not is "the enemy" as it is a blast from the past anyway of when he was a major in the army (and a very good book too)
If you are looking some background on Jack Reacher, read "the enemy" first, but if you want to start at the start Lee Childs series, then its "killing Floor" which is again a geat book i cant find fault with