I don't know if others work the same way, but my purchasing process when it comes to ebooks is rather different from that of books in a bookshop.
If I'm unfamiliar with the author, I'm unlikely to buy the first book from them as an ebook (if I'm familiar with their work, as I am with yours, it's different), and will instead look for a freebie. Then, if I like the work, I will buy further books from them, which I have done quite a few times, from Smashwords and DrivethruFiction (not Amazon as I don't have an easy way of reading DRM-protected Kindle ebooks currently, and most Kindle books on Amazon are protected).
If I'm unfamiliar with their work, but I see it as a printed book in a shop, I'm much more likely to buy it - probably because for it to arrive in a bookshop it means that a company has invested a fair bit of time and money in it. (In the past I bought two authors simply because the cover artist (Josh Kirby, before he died) was also doing covers for the late, great Terry Pratchett.)
Publishing the first book in a series that currently stands at one for free is not really a solution. However, have you considered maybe offering some of the book as a free taster? Enough for readers to get a feel for it, and be more willing to part with their money.