Nonfiction: Advanced Quantum Mechanics by J. J. Sakurai. This is actually a book about quantum electrodynamics, but it’s a fascinating read, largely because Sakurai is a superb writer. But it is full of equations, and the math made my brain hurt.
Fiction: A Purple Place For Dying. I’m re-reading all of John D. MacDonald’s Travis McGee novels, and this one was number three in the series.