If you look at the URLs of of MS's free books, you can notice the following pattern:
http://12.149.108.82/Authenticate.Asp?Id=179&Book=XXX.
Well, for the listed titles, XXX is simple
HISTORY for A Short History of Everything
CANDY for Candy and Me
LAST for Last to Die..
Well, some of the next 3 are easy to guess:
SINISTER for "The Sinister Pig"
LUCK for "The Joy Luck Club"
SHORTY for "Get Shorty"