Books are made up almost entirely of organic materials- paper, ink, glue, fibers. All these materials react to light, heat, moisture and even each other over the years, and release a number of "volatile organic compounds" (VOCs). While the blend of compounds released by any one book is dependent on the exact things that went into making it, there's only so much variation in materials.
|You all knew Jack would make an appearance :)|
Although I also have a kindle, there is nothing that's quite like the olde or new smell of an actual book.