Where do book ideas come from? They really can come from about anywhere! I read somewhere that Stephenie Meyer dreamt her Twilight saga. What a dream that must have been! Book ideas can come from the environment. I was in a bamboo forest the other day and boy did the ideas start flowing. What could live in a place like this? What if I was all alone and lost? Another muse is the past. Writing is a great way to relive the past or even foresee the future. Was there an event or time in your life that you wish had gone differently? Imagination is a glorious world where stories live. Writers give them a life of their own. Whatever idea you come up with there are two important points to remember ~ 1) make sure your ideas are truly yours and 2) devote yourself to your idea.
I make a practice to never write what I 'm reading. I can make loose notes of a future book, but I never begin outlining a new story. An example would be what I'm currently reading. I'm two books into the Twilight saga so I won't be writing about vampires anytime soon. Of course, I would never intentionally plagiarize, but my vampires would be scary similar to the Cullens and I wouldn't truly feel that my work is my own creation.
Your book idea should be your passion. It should be like your favorite pair of jeans. If anyone asked you a question about your book, then you should be able to answer it in a second. Your idea should be familiar and comfortable. By knowing in your mind how your story plays out, planning it will be easier.
Once you have an idea, write down everything you have in your head from settings to people to conversations. Just jot down notes without formalities. I, personally, use bullets. Sometimes ideas can come in a rush. Never deny a possible project so record that idea. I keep all of my ideas in a notebook. I can't be sure they'll all become something, but they are all possibilities.