There's no need to ask a writer IF he's working on a new story. The question really should be how many. A writer....wait for it....writes! Currently I've got 7 different story ideas and have started working on 4 of them.
While it's certainly a sweet, sweet accomplishment to attain a gifted critique partner, or an agent, or a publisher, or a 3 book contract, or a Christy Award, or a.... See where I'm going with this? Those things are fantastic and thrill a writer to no end, but in and of themselves they are not satisfying. There's always something more to attain. Only God satisfies.
Honestly, most authors don't know how well their current book is selling. The publisher gets the blow-by-blow numbers, not the writer. Each publisher has a different timetable for releasing that information to the author. For instance, with UNDERCURRENT, I only see numbers twice a year. With GALLIMORE, I hear quarterly.