Some useful free technical references.
Linear Algebra
Matrix Cookbook (Petersen & Pedersen)
Matrix Algebra (Minka)
Optimization
Convex Optimisation (Boyd & Vandenberge)
Supervised & Unsupervised Learning
The Elements of Statistical Learning (Hastie, Tibshirani & Friedman)
Introduction to Statistical Learning (James, Witten, Hastie, Tibshirani)
Bayesian Reasoning and Machine Learning (Barber)
Information Theory, Inference and Learning Algorithms (MacKay)
Graphical Models (Jordan)
Multi-Task Learning tutorial (Zhou, Chen, Ye)
Gaussian Processes (Rasmussen, Williams)
Algorithms for Reinforcement Learning (Szepesvári)
Time Series, Econometrics
Notes on Time Series (Weber)
Econometrics (Hansen)
Machine learning vs. Statistics
Glossary (Tibshirani)