Is it a series? Or stand alone? If its stand alone, based on what I have read/told, your backlist does not have much effect until you have 4 or 5 books on the market. If it is a series, it could have big impact.
In my case I did not see an uptick at all in book 1 or 2 when I released the next book. Sales are pretty steady, with book 3 still outselling book 2, but I am writing a series. I suspect when I release book 4, and the 4-book omnibus edition that will end the first series that will help sales. I'll let you know if you remind me after book 4 comes out.