If you are very lucky, an idea for a book will arise out of (seemingly) nowhere, floating into your brain as you awaken, or are driving, or washing dishes. The idea may be vague and meaningless to everyone but you, especially when written down. And you probably won't even be able to call it an "idea", except that you can tell by the charge it gives you, it may grow into one. Especially if a bunch of "what if's" are attached to it. It's actually a book "concept".
WRITE IT DOWN QUICKLY (on that napkin, bedside writing pad, parking stub), then scribble some notes in order to explain the gibberish to the future you, who will have to work with it
THE FIVE LIVES OF OUR CAT ZOOK began this way. "Cats have nine lives" I wrote on my own bedside writing pad, an adage I'd heard all my life. But this time it had special potential to me as my next story. What if a kid really believed that? What if each life was a separate story?
Pay attention to these little slips of half-formed ideas. I once read that Roald Dahl dashed off a note to himself--something like "strange man owns candy factory". We should all be that lucky, not to mention gifted...