CS229 Lecture Notes: Unsupervised Learning, Andrew Ng, 2018Stanford University CS229 Machine Learning Course (Stanford University) - Official lecture notes from a respected machine learning course, offering a theoretical introduction to unsupervised learning algorithms, including clustering.
An Introduction to Statistical Learning: With Applications in R, Gareth James, Daniela Witten, Trevor Hastie, and Robert Tibshirani, 2021 (Springer) - An accessible textbook introducing statistical learning methods, with a chapter on unsupervised learning covering clustering and related topics.