I am glad I have the opportunity of rating the first 6 books as a bundle, because I am reading them so quickly I am not entirely able to separate them.
Jack Reacher was born in an US army base in Berlin of an officer father and French mother, moved all over the world as a child and then joined the army himself and became an MP, an army cop. When the series begins, however, he has left the army and decided to wander, not even carrying a bag, owning nothing and as free as it is possible to be - no cell phone, no email, no responsibilities, no paper trail... He even buys clothes every couple of days that he then thrashes, so as not to have to clean and own them.
And Child makes it work, which is in itself very thought-provoking.
Reacher is huge (which makes Tom Cruise impersonating him problematic), extremely strong and skilled in combat - hand to hand, guns, multiples, strategy, and even knives although he doesn't like them. He is ruthless, sometimes coming quite close to the psychopath line, but hey, he does have soft spots and he is EFFICIENT. He gets things done, he doesn't allow sentimentality, shock, emotions to get in the way. In the way of his thinking, that is, because he is intensely smart, and thinks like a cop, all the time.
So take a guy with his impressive (but limited, in his own words) skillset, cut him loose on American roads and see what happens.
Because another great character in the books is America itself. Reacher said he spent a total of 5 years in the US as an adult and he now wanted to know his country better. Each book takes place in a very distinct location, and those really stayed with me, from the 'Pleasantville/Edward Scissorhands' eerie little town in the first book, to the cultish mountainous boondocks in the second, to the Red House in the stifling heat at the Mexican border, to the streets of New York city... Child, who is in fact British has done his homework very well. The locales stayed with me even more than the other characters.
The storylines are incredibly well built, progressive, detailed yet fast-paced, with many things at stake, or balls to juggle with. And Reacher remains mercifully calm throughout. I must admit I got a little tired of his being mistaken for the bad guy, arrested or harassed in the first books, but that streak eventually stopped, and I guess his lifestyle is so uncommon most cops would lock him up and ask questions later.
Also, you will spot what kind of woman Child goes for in every book; she is usually small, thin, blonde, athletic, perfectly shaped and wears A-shaped sleeveless dresses. In literally every single book. But hell, let the guy have his fantasy!
Note: I am a woman, and most absolutely liberal, btw, and I love the books - this in answer to a couple of comments to the effect the books were addressed to a male gun-loving population....