@coastalguy-
Aside from The Roku Channel, Roku Media Player, and a few Roku information channels, Roku doesn't create the channel apps, the channel owners do. For the most part, Roku just provides the platform on which the channel apps run.
You could contact Amazon and request they develop a Kindle reader app for Roku, but I doubt they would be receptive. It would not be a normal Roku app as it would have to read the book file from your computer or other device instead of just dealing with it as it is streamed in from their server like a normal video file.
Your best bet is probably to learn how to cast or mirror from whatever device you use to read your Kindle files to your Roku.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X