I have seen marketing services for kindle book here on this forum. I suggest you try using th search feature to locate some of those service as that may help you

Check this out
Amazon's new way to boost Kindle sales? Your old books - Fortune

You can increase the rate then make a discount offer. such as your actual rate is $100.00, now you have to make the rate is $140.00 then you can make 40% off. After giving this discount, you rate will be $100.00 so there is no effect on the main rate. You can try to apply this formula. Thanks

Was it the old ebook's sales that plummeted or the new one?

John

You may want to consider joining the Warrior Book Club here on WF, as there are many guides available on publishing evergreen content with a high potential to sell.