I teach French literature so there are some Great Books that I must deal with all the time, like Pascal's Pensées or the Essays of Montaigne, as well as some plays by Racine and Molière.
The fabulous memoires of the Duc de Saint-Simon, the great chronicler of the court at Versailles under the Sun King, are a favourite, an endless source of delight on rainy days and bummy hours. Twelve huge volumes printed on Bible paper!
Saint-Simon had a decisive influence on all writers attempting to re-create an entire world, a whole period in history and the people who populated it, from Tolstoy and Proust to Henry James, Faulkner, Churchill, Bruce Catton and many others.
I also love Tolstoy. His great three novels, the even greater novellas and short stories, his plays. But a book I can't do without is his War and Peace. My grandparents were Russian émigrés and they loved this book above all others. I can still hear my grandmother's voice reading it out loud and explaining it to us children. It's "truer than truth itself", as Nabokov said.