This and Speech and Language Processing
by Jurafsky and Martin are the two big introductory texts in natural language processing. I prefer the Jurafsky book; it goes into more detail, has more examples, and is written more for use as a class text. The Manning and Schutze book is much more mathematically oriented and goes into more detail on algorithms, so if you're focusing on the statistical aspect more than the language aspect, refer to this book. Ideally, you probably want both.