How do I write an annotated bibliography of a book?

My teacher wants us to write some annotated bibliographies of articles, web sites, and books, but how do I write one for a book-because it seems to me, that I would have to read the entire book so I could write a annotated bibliography for it, but I don't have the time, 3 days. To read 300 pages? It could be done, but I have to finish the rest of the ignment as well. What do I do?