These books should be usable on any Kindle or Kobo device. Just be sure download the appropriate format for the device - the download page on this site will tell you which one you need for your device.
Personally, I use a Kobo Libra 2. Not the cheapest ereader at $179 USD, but I went with it due to USB-C, IPX8, and support for the third-party KOReader viewer.