The 39 Clues series is currently maintained by Scholastic, with them choosing the next author. The storyline is pre-decided, and each author writes the next part of the plot. For example, Scholastic came up with the outline and Rick Riordan fleshed out the main storyline and wrote the first book, The Maze of Bones.
See here for a slightly old (but still accurate as far I as can tell) explanation.
Here's a quote from that page:
Who is writing The 39 Clues books?
Rick Riordan, #1 NYT bestselling author of the Percy Jackson series, kicks off the action with the first book, MAZE OF BONES, which introduces the main characters, sets the tone for the series, and reveals the first big clue. Riordan is also the architect of THE 39 CLUES story arc; he’s created the entire plot outline for the central mystery. Millioncopy selling author Gordon Korman (Island, Everest, Dive) has written Book 2.
Acclaimed author Peter Lerangis (The Watchers, Antarctica, Spy-X) will write Book 3. Jude Watson, New York Times bestselling Star Wars Series author will write Book 4. Patrick Carman, author of the internationally bestselling Land of Elyon series, will write Book 5.