Essentially, a great book is one that grabs you on the first page and doesn't let go, not even at the end, because you walk around thinking about it for the next couple of days.
A great book also is written with beautiful language. The author knows how to play with words to create sensations that go beyond the story. The words make you fall in love with language. This can happen in any language, and even in translations. Few novels are as beautifully written as One Hundred Years of Solitude by Gabriel Garcia Marquez, but my Spanish isn't good enough to read it in the original.
A great book has characters that make you empathize with them, even if they are nothing like you. When Sydney Carton sacrifices his life for his love in Charles Dickens's A Tale of Two Cities, it touches my heart. "It is a far, far better thing . . ."
As a former English teacher, I am attracted to books with a strong universal truth. Some of my favorites are Toni Morrison's Song of Solomon, Isabel Allende's House of the Spirits, and Kate Mosse's The Winter Ghosts. And a new one with a strong theme of family and the secrets we all keep in The Hiding Place by David Bell.
A great book needs to stand the test of time. It may become dated, but there is still a truth within it that means something to readers. I think of J.D. Salinger's The Catcher in the Rye. It never failed to delight me when high school sophomores, even within the last ten years, would say how they loved Holden Caulfield, and he reminded them of themselves.
A great book can be old, new, an established classic, or a genre book like The Hiding Place.
What are your "best books?"