I have now made the Unseen Things series available for purchase directly here at the official website. It’s still available at the other online retailers as well. Unlike the other online retailers, when you buy direct through the website, you’ll receive download links for the Epub, Kindle (mobi) and PDF versions of the books rather than just one specific format as you would on the other sites.
Just click the Buy the Books! nav link at the top of the page.