Fantastic Bureau of Imagination by Brad Montague

Every day, special figment agent Sparky delivers all the mail the FBI receives to the proper department, like the Office of the Unexplainable or the Department of Dreams. It's a big job, but Sparky keeps everything running smoothly . . . until disaster strikes when the Cave of Untold Stories overflows and threatens to topple the whole bureau. It turns out too many people have been holding in their big ideas, and now Sparky must recruit more agents to share their dreams, songs, and stories with the world.