Textbooks

• Speech and Language Processing by Daniel Jurafsky & James H. Martin (Chapters on N-gram models and smoothing)
• Foundations of Statistical Natural Language Processing by Christopher D. Manning & Hinrich Schütze

Video Lectures and Online Courses

• Coursera: Natural Language Processing Specialization
• YouTube: N-gram Language Models and Smoothing (Stanford NLP)

Research Papers and Articles

• Chen, S. F., & Goodman, J. (1996). An Empirical Study of Smoothing Techniques for Language Modeling. Proceedings of the 34th Annual Meeting on Association for Computational Linguistics.
• Wikipedia: Additive Smoothing
• Wikipedia: N-gram

Online Resources and Tools

• NLTK Documentation: N-gram Models
• CMU Statistical Language Modeling Toolkit
• Google Books Ngram Viewer

Additional Reading

• A Gentle Introduction to Smoothing in Language Models
• Stanford NLP: Language Modeling