To be honest with you rosearche I don't think it would be a waste at all. Even if the the first book isn't very well received commercially there are still alot of benefits to be had from the research and writing of the next book. Even if all you do is visit New York, by all accounts a very nice place with alot of culture and history (as historical as America can be, anyway) and the experience of completing another piece of work that you yourself can be pleased with.. Very tempting stuff.
Some writers can go their entire lives without even completing a piece of work that they feel they've done right by, the fact that you've done just that is wonderful thing by itself. If you want my advice just go do it. Chase the idea until it's all in your hands and you're happy, so to speak.
Or you could not bother? Write some freelance "Do's and Don'ts" for a skincare magazine. Is there a hanging emoticon here..
BTW, would you mind sending me a little teaser of your work? Maybe a chapter or something? Maybe I could give you a more educated opinion having seen the story in question.
In any case - BEST OF LUCK!