Back in the 1980s, when Boston bands like Aerosmith, The Cars and J. Geils were topping the charts nationally, a flourishing ecosystem of not-quite-so-famous groups were struggling farther down the pyramid. "There were another 2,000 bands trying to claw their way up the food chain," said musician and author Terry Kitchen, whose new book and CD, "Next Big Thing," is set in that world. Kitchen writes from experience. And he'll be delivering a book talk and playing songs Thursday night at the JP Branch Library. And while the Boston scene as a whole is the star of the narrative, JP plays a role near the heart of this rock n' roll story.