I'm in the middle of the "Wheel of Time" books. That must have been the most planned out series ever. Or reworked like a million times. At least, that's what I gather from people who have reread the series and praise the amazing foreshadowing in the first half of the series (even books 1 and 2).
For those who don't know, "The Wheel of Time" is 14 books plus a prequel.