What are some good graduate-level econometrics books for someone with a strong mathematics background?
"Adult" Wooldridge is great intro to various microeconometrics topics.

For time series, Hamilton's Time Series and Lutkepohl's Introduction to Multiple Time Series Analysis are both nice, though Hamilton is a bit dated and Lutkepohl is more focused.

As far as more foundational, rigorous material, Herman Bierens has a short Introduction to the Mathematical and Statistical Foundations of Econometrics. Gourieroux and Monfort have a whole flock of graduate econometrics texts, but to quote an anonymous reviewer, they are "in the French tradition of excellent precision and terrible pedagogics," though they have their champions.