Long series? Well some long series just roll on and on as revenue generators long past their expiration point but some do make it to double digits, with the same cast, without wearing out their welcome. Two that come to mind:
David Weber's Honorverse - Starting with ON BASILISK STATION, 14 novels (and counting) and back-filling with a few short stories afterwards he is chronicling the life and times of the premier military officer of her era, starting with her middie cruise to her leading role in a galaxy-wide epochal war.
Robert Lynn Asprin's - Myth Adventures - 19 volumes and a great comic series of fantasy comedy-adventure starring Skeeve, a magician's apprentice turned demon's apprentice. http://en.wikipedia.org/wiki/MythAdventures