Upon further research, the 75% will apply to Kindle books on the Amazon most read or most sold book list.
Any other Kindle book will give you a 30% credit.
https://www.amazon.com/Kindle-eBooks...node=154606011
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I was browsing through kindle books and saw this notice below the price for the book I was looking at:
Prime Day Promo: Prime members get 75% of the purchase price as credit toward their next Kindle eBook. Only valid on the first eligible purchase. A confirmation email will be sent once the credit has been applied.
I think the % credit is different for different books.