The first book in a long series was The Hunter. This a bare bones revenge story. Parker is betrayed by his partner and wife right after pulling a very profitable heist. The story is so compelling it was made in to a major motion picture three times.
First by John Boorman in 1967 as Point Blank. Other outstanding films by Boorman are Deliverance and Excalibur. Lee Marvin is suburb and really can say a lot without saying a word. For some reason, Parker was renamed Walker in the film.

Lastly, it was remade in 1999 as Payback. Mel Gibson took the Parker part and generally growled an imitation of Lee Marvin. The producers tried very hard to give this a '70s look and feel and succeeded for the most part. In this film, Parker character is named Porter. The studio was originally unhappy with the first cut of this movie and hired a new director to rewrite and re shoot about 30% of it for a lighter, comedic tone featuring a particularly mean spirited Gibson. Only his star power at the time prevented this from being a really bad idea.
By far the best adaption is the Lee Marvin version. It plays a little dated now but keep in mind that California was a very interesting place to be in the '60s.
The Parker character proved to be rather popular and inspired a further twenty-three novels. They are somewhat in sequence in that characters may be introduced in one novel and continue for a few books. Sometimes they exit nicely, sometimes they don't. The Parker novels were written over a period of forty-five years and have been re-issued several times.
Their popularity has spawned graphic novel versions of the first two books in the series. They are amazing even for the people who have read the originals.