Eventually! After years of promising Charles Barber (a conductor himself) has published his book: "Corresponding With Carlos:  A Biography of Carlos Kleiber"

It is available through Amazon

http://www.amazon.com/Corresponding-Carlos-Biography-Kleiber/dp/0810881438/ref=sr_1_4?s=books&ie=UTF8&qid=1304981639&sr=1-4

OR, for some reason cheaper in Canada,

http://www.amazon.ca/Corresponding-Carlos-Biography-Kleiber/dp/0810881438

This promises to be the most interning book of the year! Charles Barber was in absolutely unique position and for years exchange letters and ideas with great Carlos Kleiber – something that no one else ever did. I can wait to read the book.