The Dragonfire Novels are currently enrolled in KDP Select and you can read the series free in Kindle Unlimited. This is their last 90-day term, though: they will leave KU and be republished at all other portals at the end of October 2022. The end dates for the term vary between the books but they’ll all be available elsewhere by the middle of November. Once they’re republished, Kiss of Fire will be a free series starter again.
If you’re a KU reader, it’s time to get through the series!