For Kobo readers, the first book in the Vistaria Has Fallen series, which is also called Vistaria Has Fallen, is free today and until April 22nd (Sunday).
The Kobo promotion page is here, and Vistaria Has Fallen is listed under the Thrillers and Mysteries.
Here is more detail about Vistaria Has Fallen, including an excerpt.
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For a short while, you get a bundle of ebooks, free, when you sign up, as a Starter Library. Details here.