Although a series, all of these Reacher novels functions well as a standalone story. I recommend beginning with the first book, "Killing Floor," or "The Enemy," which was a prequel.

Parker is best known for his Spenser series of books, which has spanned more than 30 years, and about as many books. The eponymous main character was a radical departure from the brooding, angst-ridden private eyes of the generation before. Here was a confident, wise-cracking dick who rarely lost a fight, and who spent as much time pondering the ethical implications of his own actions as he did breaking some punk's nose.

As with most series, it's best to start at the beginning, but even moreso with Parker, as the quality of his work has not improved over time. The early books are all excellent starting points, but after, say, the fourth book, Parker begins to include spoilers to previous stories. Start with _Godwulf_, _God Save_, or _Mortal Stakes_.