Since people keep talking about how a book needs to be a certain length or more (from here
http://commonplacebook.com/art/books/word-count-for-famous-novels/):
42,715 – The Tequila Worm – Canales, Viola
46,118 – Fahrenheit 451 – Ray Bradbury
47,094 – The Great Gatsby – F. Scott Fitzgerald
47,180 – The Red Badge of Courage – Stephen Crane
48,523 – The Outsiders – S.E. Hinton
49,459 – Slaughterhouse-Five – Kurt Vonnegut
54,243 – The Hours – Cunningham, Michael
56,695 – As I Lay Dying – William Faulkner
56,787 – A Separate Peace – John Knowles
58,428 – The Wind in the Willows – Kenneth Grahame
59,635 – Black Beauty – Anna Sewell
59,900 – Lord of the Flies – William Golding
60,082 – The Dew Breaker – Edwidge Danticat
61,922 – All Quiet on the Western Front – Erich Remarque
63,422 – Mrs. Dalloway – Virginia Woolf
63,604 – The Scarlet Letter – Nathaniel Hawthorne
63,766 – Brave New World – Aldous Huxley
64,768 – The Martian Chronicles – Ray Bradbury
66,556 – The Color Purple – Alice Walker
66,950 – Treasure Island – Robert Louis Stevenson
67,203 – The Fault in Our Stars – John Green
67,606 – Ironweed – Kennedy, William
67,707 – The Sun Also Rises – Ernest Hemingway
68,410 – Drinking Coffee Elsewhere – Packer, ZZ
69,066 – The Adventures of Tom Sawyer – Mark Twain
70,957 – Woman Warrior – Maxine Hong Kingston
72,071 – White Fang – Jack London
73,404 – The Catcher in the Rye – J.D. Salinger
77,325 – Philosopher’s Stone – JK Rowling
78,462 – The Picture of Dorian Gray – Oscar Wilde
A book is whatever length it should be. It's possible that you might get an agent that doesn't bother to read it unless it meets x words, but do you really want that agent that doesn't even read the query or the first ten pages because of an arbitrary word count?