The Jack Ryan books by Tom Clancy would also fit into this. They just keep going and going...
There are quite a few Jason Bourne books too. The first 3 are by Robert Ludlum, the remainder are by Eric Van Lustbader?
I saw WEB Griffon mentioned, he has a couple fairly long series, not super long, but 5-7 books I think.
The 1632 series by Eric Flint (Ring of Fire?) also has a lot of books and is still going. I haven't read it yet, so I don't know how much revolves around a central cast though.