One of my favorite feminist books is Herland by Charlotte Perkins Gilman. It's basically a novel about a feminist utopia, and is a quite unique read. I also really like Stone Butch Blues by Leslie Feinberg, but that might be a bit more of an LGBT studies book than a women's study one. A good mix of cultural and women's studies I enjoy is A Taste of Power by Elaine Brown.
Borderlands/La Frontera by Gloria Anzaldua was enlightening when I read it.
Anzaldua also edited This Bridge Called my Back which is an amazing collection of essays.
I'm also very partial to the contributors of INCITE! Women against violence...especially Andrea Smith.