I've now been published for 2 months. I haven't had a full royalty statement yet, so I have no idea how many books I've sold (not a whole lot, I suspect), but I have found various ways to figure out if people are even interested.
Firstly, there's Goodreads. I pop in there every few days and every time more people have added An Unstill Life to their TBR piles or whatever they categorize them as. Which is good. Especially when they're not people I know in real life or through the online writing community.
I check the book's page on all the sales sites for reviews too, but there aren't a lot. Whether that's because people don't like leaving reviews, or if it's because no one is buying it, I don't know.... If you buy a book from any retailer that allows you to leave reviews, even just a line or two makes a huge difference. It's hard not knowing if anyone is reading it.
The the best thing I've found, is that by Googling my title and my name, I can see if anyone has mentioned my book anywhere. I've found several blogs where readers have listed my book as being one they're looking forward to reading. And that's really exciting! Of course I have no way of knowing if they actually buy the book unless it's mentioned again later, but just knowing that the blurb/cover/gossip has made them want to look further is cool.
There are sales tracking sites like Novelrank too, but because they only track sales from certain Amazon retailers, I don't think it's a 100% accurate indication of sales. But maybe that's just denial on my part because I don't want to believe the numbers I see there....
Do you have any other ways to gauge interest in your book? Please share!