Basically, every idea I get is theoretically a book idea. I haven't written a short story in a few years now; novels were always my real writing love. The question is more, when is this idea ready to be in a book.
I jot down any idea that strikes me as a great premise, or an interesting character or world-building element, or even an intriguing line of dialogue or description. These first end up in the notebook I carry around with me everywhere (I usually get the five-section ones, with the first section for random ideas, three sections for whichever WIPs I'm most focused on at that point, and the last for non-writing notes like to-do lists and day job stuff).
Before, if I was looking for an idea, I'd have to page through various notebooks (I have 12, going back about seven years) trying to find where I'd mentioned it. Recently, I've started transferring all these ideas onto index cards which I keep in a box on my bookcase, separated into categories: Full Ideas, Plot Tidbits, Characters, and Misc (which contains world-building, setting, dialogue, etc.).
The Full Ideas section is for ideas that I've already done a fair bit of outlining on. Some ideas come and just sort of sit there. Others come with a whole bunch of extra inspiration attached--oooh, that's a great premise, and then the main character would do this, and there'd be this other character who showed up that way, and in the end they'd have to... In my current notebook, I have about twenty pages devoted to my current idea-darling, a book I will call "The Japan Book" here because my actual working name for it is spoilery.
These ideas are rarely ready to be written as an actual book right away, even the super-inspired ones. I'll write down everything that comes to mind, and stew over the idea, and write down more. Then I'll get distracted by the ideas that are ready to be written, and leave the others for a while, and come back to them later and stew some more. So my notes end up spread over more than one notebook. Which is one way the index cards come in handy--I've numbered the notebooks, and noted which notebooks have notes on each Full Idea (and where approximately in the notebook they are) on the index card, so I can find all my thoughts fairly quickly.
How do I know when an idea is ready to become an actual book? Well, I need to have all the major plot ideas in place--particularly, I have to know how it ends. (I have one book idea I really love and have been stewing over for a few years, but I don't have the right ending yet so I can't write it. :( ) I have to be able to sit down and outline the story and make it through with no major holes. And I have to know why it's important to me to write this story. If I'm not sure yet what the story's really all about, it's not time yet to write it, and any attempts I make are likely to fall flat.
Often, it's just a matter of periodically coming back to the ideas I love and poking at them to see if anything new stirs in my head. Eventually, the missing pieces just fall into place. I do a lot of active brainstorming, but I find, most of the time, it's impossible to *make* an idea be ready. That just sort of happens. Thankfully, it happens often enough that I'm rarely without something to work on. :)