I actually found that in my college's library after I saw the movie. The youngest one of the three, the one who didn't get executed, wrote the book.
It's funny, on Wikipedia they imply that the book was republished in 1982 because the original, published in 1907, had pretty much disappeared. The copy I read was definitely older than 1982.
Originally published in 1907 and was reprinted in 1982 by Angus & Robertson.
Many 1982 versions available.
Did you find an original copy in the school library? Where? (Yea, I know. Under "History.")