It doesn't matter how much traffic you get if none of those clicks are from people who are ready to buy your product.
The Kindle format is a killer for me: I see it and I want to click away and never come back. Let me guess a few reasons why.
The price is obviously a signal of low quality.
Kindles are a pain to work with when I'm sitting at a computer (which nearly 100% of Wordpress-plugin authors will be doing). I have to click the link. I have to dig out my book reader. I have to avoid getting distracted by various novels. Just sell me a PDF like everyone else does. Check out Gumroad or getdpd.com. (I've never used these, but folks in the 30x500 group speak highly of them.)
(Yes, I know there are people who read Kindle books on their PCs, using the web client or the PC client or whatever. Do you want to limit your market to these people? All of us have PDF readers on every device, and use them.)
Amazon is the wrong neighborhood for you. It's full of professionally-edited and marketed books with real covers that make yours look silly and yet sell for a dirt-cheap $9.99 (they make it up in volume), which anchors your price to a low value. And Amazon is also stuffed with absolutely lousy vanity-published dreck, which makes me leery of buying anything on there that doesn't look like a professionally-marketed book.
On Amazon people literally judge books by their covers, just like we do in a physical bookstore, because the cover is generally the most prominent, best-designed thing on the Amazon sales page. You could pay for a professionally-designed cover, but that is the wrong move: You do not want to compete in that market. Just go sell in a venue where nobody cares about the cover because they're too focused on the goal. A venue like: Your website about Wordpress plugins.
There is no magic trick to selling a book on Wordpress plugins. You set up a website. I presumably don't need to tell you what tool you could use for that. On the website, you put up many short articles about building Wordpress plugins. On every page you put a tiny form that says something like "Do you need to build a Wordpress plugin? Get a free sample chapter of my book 'Creating Wordpress Plugins': [form field labeled "email address"] [button labeled 'Get Sample Chapter']".
How much of your book should you give away for free, a few paragraphs at a time? Well, Michael Hartl gives away 100% of his book for free at http://ruby.railstutorial.org and still does okay. I'm not sure that's the optimal strategy for you, because that depends on your goals, the size of your audience, how ready the audience is to buy, whose money the audience is spending, the amount and kind of value that you provide, and so on. But it's good to understand that your content isn't some kind of precious jewel that you need to hoard.