This is exactly the type of thing that I would use given that I do not have an iPhone (or something similar). Even if I did have an iPhone, I think the convenience of punching in the ISBN and getting a direct answer trumps having to launch Safari, navigate to Amazon.com, do a search, select the title, pinch (zoom in/out), etc.
I'm definitely going to try this one out (and this is coming from a hard core skeptic!).
I would suggest adding an SMS interface. Also, if I could get an email reminder of the books I was looking for then that would be pretty cool as well. Also, if I could get some information about discounted bundles then it would be a sure thing for me to actually use regularly.
I agree with JAcobAldridge's comment about a dwindling market. My suggestion in response to this would be to build an iPhone/Android/[whatever] app next and keep all channels open.