Show that you are studious by providing evidence that you completed the assigned reading or research. First, however, here are obvious signs that you did not do the reading: (1) you misspell an author's name, the title of a book, or the name of someone listed in a book; (2) you seem to have guessed what the book was about on the basis of the title and the first few chapters; (3) all of your quotations come from the same chapter or even the same page, or your quotations or main ideas come from ClassicNotes or another reading guide; (4) parts of your essay are plagiarized.
(1) Make sure that all proper names are spelled correctly.
(2) Read the book. If you have run out of time and cannot get an extension, look for a table of contents and an index. Learn what the book is about from these sources and from online reading guides such as ClassicNotes. You could choose a topic that involves the development of a theme throughout the book.
(3) Many books are online as etexts. Try to find suitable quotations from throughout the book by searching for a theme word in the book's index or table of contents or by using an etext.
(4) Do not plagiarize. You might be surprised how easy it is to get caught plagiarizing, and the consequences can be extreme.
Beyond that, note that in general, novels have a plot, poems have a subject, social science texts, philosophical works, and works about the humanities have an argument, scientific and many social science texts have a finding, textbooks and technical writing have information, travel writing has observations, and political and business writings often have action items. Make sure you can state what the central points are of whatever you have read.
Note that if you try to put a large number of the book's central points into your essay, the essay will seem uncontrolled because it ranges so widely. You can show you did the reading by tracing just one or two topics through the course of the book.