Mathematical Methods in Statistics: A Workbook

This is a companion to the very good introduction text Statistics by Freedman, Pisani et al. If you have the least bit of math background, you must look at this book as you read the main text. Still, there is a lot of room for at least mentioning the more serious mathematical treatment of the subject. This is because there are quite a lot of readers who are very comfortable with undergraduate mathematics, but know absolutely no statistics, and mentioning the underlying mathematics would make things a lot more clear.