In addition to right here on the official site where you can get the books in Epub, Mobi and PDF format, you can purchase the Unseen Things series from the following online retailers:
Google Play (Epub)
OmniLit (Epub, Mobi, PDF)
Kobo (Epub) – The best link I can provide. They don’t have author specific listings.
Barnes and Noble (Epub)
The ebook subscription service Scribd also has the series available to read, though you need a subscription to be able to view them.
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You can buy these standalone novels at the following retailers as well: