Michael Oakes 
Statistics for Corpus Linguistics [PDF ebook] 

This book in the Edinburgh Textbooks in Empirical Linguistics series is a comprehensive introduction to the statistics currently used in corpus linguistics. Statistical techniques and corpus applications — whether oriented towards linguistics or language engineering — often go hand in glove, and corpus linguists have used an increasingly wide variety of statistics, drawing on techniques developed in a great many fields. This is the first one-volume introduction to the subject.