Nicola Griffith wrote the classic novels Ammonite and Slow River, so she's uniquely qualified to weigh in on lesbian-themed science fiction books. Someone asked her for some recommendations on her blog, and her list is well worth checking out.
Including some obvious choices, like James Tiptree, Jr. and Joanna Russ, but also some books you probably haven't heard of, like Kelly Sinclair's In The Now. And the recent anthology Heiresses of Russ, whose cover is featured above. Check out the whole list at the link. [Ask Nicola]