Before you can start writing a book you need to have a book idea. Listen to Robin Rices's advice on crafting a book idea:
Now, for your Writing Prompt: Brainstorm as many book ideas as possible. Don't edit your thoughts. Be creative. Ready, Set, Write!
Now, time for some collaboration, take your list and share it with someone. Have them tell you which idea they really like and why. See if you can develop this idea into a novel.
Have a Creative Weekend!