Stylometry is the quantitive study of literary style.
It is based on the observation that author tend to write in relatively consistent, recognizable and unique ways.
Here, we have a collection of some written pages of some authors and also some are from unknown and also some are written by more than one author.
We have to analyse the style of all diffrent authors.
Well it's done through knowing the word length distribution.
There are more advanced concept than this like:-
Kilgariff's chi-squared method
John Burrow's delta method