I'm currently writing a novel about the life of Matthew Fontaine Maury and am researching through five different biographies. I've decided to re-read some of my favorited authors to catch up for the next book in their series due to come out this year. They're like curling up on a couch in front of a fire and having a chat with old friends.
I just finished the 54th book, The Killer, in the Casca Eternal Mercenary series. Tony's written now 30 of them. In this one he's fighting in the Algerian Civil War during the 1950's for the French Foreign Legion. I'm not re-reading the other thirty books, they're stand alone.
There are currently five books in the Dark Elvin series: Dark Blade, Heir of Gorridan, Okra's Tower,Faerown's War, The Mountains of Butchock. A new one is due out this year. The last few books Roberts has unleashed Faerowen's sensual side.
The Kastania Chronicles are by far the best fantasy series I've ever read, even better than Tolkien and C.S. Lewis. They now comprise Prince of Wrath, House of Lust, Patch of Pride, Throne of Envy, and God's of Gluttony. I can't wait for the next one coming out this fall.
All told there's many more than ten books here, but I count each series as one extended book.