Right now, you can download AURA for FREE at Barnes & Noble, iBooks, Kobo and many other online bookstores! Go here to select a store. Unfortunately, Amazon has not price matched to make the book free (if you go to Amazon and tell them that you found if for a lower price elsewhere, they might.) However, AURA is still only $0.99 on Amazon. All of the other books in the series can be downloaded for free if you are a member of Kindle Unlimited.
Best hurry, this free offer will last not last forever.