You can purchase the Unseen Things series from the following online retailers:
Amazon (Mobi, Paperback)
Google Play (Epub)
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: