As part of my unending search for ways to market my book, I've discovered an quick way to increase sales by 10% (those are my results, yours may vary) for the rest of my series. Simply, insert a hyperlink into your Amazon books linking your other Amazon books. This really only works for Amazon as Smashwords doesn't allow hyperlinks.
You must have multiple books** in your series in order to do this because it a trickle down sales effect. Once your readers read and like your initial book, they'll want an easy way to get more. That is the reason it works, a subsection of your sales want the easy way to get it so having the link in your books is a perfect way to get your next book(s) almost instantly.
** special note: if you only have one book in your series, I recommend you partner with another author with similiar sales and genre and swap links.
1) First backup your eBook.
2) Highlight the text you wish to hyperlink. I create an "other books in my series" page in all my books.
3) Right-click and select hyperlink.
To see a live example, download a free copy of LUNARA: SETH AND CHLOE from Amazon.