Ye they probably use the russian Antonov AN-225 now that america/russia are buddies in space!! Nasa did use a specially adapted Boeing 747 for piggy-backing originally
Abort modes and sites are covered in Tim Furniss's Book "The Story of the Space Shuttle".
He covers all about the abort modes and sites in the book and is very interesting book on the history of the shuttle all the way from the X-15 to the Challenger disaster.
  