Well, the classic way is to set up a web site describing the book, which has a "Buy" button. If/when people click, you explain the the book is not available ye

Well, the classic way is to set up a web site describing the book, which has a "Buy" button. If/when people click, you explain the the book is not available yet, and collect their email address to notify them when it is available. Then, you pay for some Adwords traffic (say, $25 worth) to drive people to the site, and see what your Conversion Rate is. In your case, though, if you already have a website in that niche that gets decent traffic, there are probably simpler ways to take the pulse of your current readers.