The series is currently free. I believe it's free for any Audible Plus members (like me). At least I would assume you would need to be a member to listen to the series for free.
There are 11 books in total. The first one was published in 1974 and the final one was published in 1994.
I've been listening to the novels. They are pretty good. They are also short books and you can get through them pretty quickly. I just finished book 3 (Fletch's Fortune). I really liked the first book. The second two books have fallen off a bit for me.
If you're an Audible Plus subscriber and looking for some good books, check these out.