Bookstore ideas stand up over time, are typically high level (like, "The Tipping Point") and might change the way your audience thinks. Bookstore ideas are akin to purchasing a book from a bookstore because you plan to reference the idea occasionally, like the ideas in The Tipping Point.
Library ideas evolve, impart specific knowledge and are typically tactical (like, "Survival is Not Enough"). Library ideas are akin to taking a book out of the library to learn a specific bit of information, like a new method for making bread. There's no need to buy the book because you'll remember what you need after the book is returned to your library.
To come up with one bookstore idea, you'll come up with some library ideas and, as Seth Godin says, a lot of bad ideas.
How does your idea generator work?