Okay, answering this could get weird, you have been warned! :)
Sometimes, ideas for stories just walk into my head. They arrive as fully formed characters in situations that are out of their control. This often happens when I'm doing something else - like driving. And is also the reason I often end up somewhere I never intended to go!
Take my latest book, Rage. I thought I had Callum's life all figured out. I knew who he would fall in love with. I knew the story line. All I had to do was write it. So I sat down to do just that, but instead of writing what I'd planned, four women appeared on the page standing around a dead body, and I knew Isobel was the woman for Callum. Where did this come from? I have no idea. But I think there's a version of Invertary's evil Betty living in my subconscious. This woman has a big stick and she stirs up everything that's gathered in my mind, once it's swirling, she dips her hand in, grabs something at random and throws it at me - and that thing becomes a story.
Told you this would get weird!
My first romance book, Mad Love, started with a couple of lines that popped into my head one night when I couldn't sleep - "Icarus was an idiot. Everybody knew that. If you are going to fly to the sun you don’t use wax to stick your wings together." From there, the story followed.
For Lingerie Wars, I was feeding the chickens while I thought about alpha heroes (as you do!). I thought, what's the least manly situation I could put a very alpha hero into? That's when my preschool daughter shouted that the dog had run off with my underwear again. (When she was a puppy, our dog often stole my bras from the washing line and deposited them around our neighborhood. Yes. That is as strange as it sounds.) I looked at the dog, with a brightly colored lace bra in her mouth, and grinned. What if this ex-military guy had to run a lingerie shop? And Lingerie Wars was born.
You might have guessed by now that there is a lot of useless information stuffed in my head. It's a huge melting pot filled with everything I've read, seen and experienced. And with Betty stirring it up, things constantly get paired with other things they normally wouldn't get paired with. Like an old woman on a holiday to Peru who steals a mummified body. This came from a chance meeting I had years ago with a very proper English octogenarian, who'd randomly decided to stop knitting and backpack through South America instead. I met her in Bolivia, where I was working at the time. She was an amazing woman, who stuck in my mind. Combine my memory of her with a book I read about mummies and voila, you have Relentless!
This random pairing works for other books I've written too. But sometimes, a book is based on something I've done myself. Years ago, after reading far too many crime novels (Sara Paretsky, I blame you for this next bit!) I interviewed for a job as a private investigator. My qualifications for the job were a degree in fine art, a slew of random part time jobs and a voracious appetite for detective novels. I didn't get the job. But, when I came to write Reckless, my experience fed into Megan's character and she became a woman with no skills, but lots of enthusiasm, who suddenly works for a security company.
As you can see, book ideas come from everywhere. Every conversation I've had, every random thing I've done, it all feeds into the massive, complicated, resource I have inside my head for story ideas. You never know, answering this question might be the instigating point for a book in the future.