I have a Kindle and I still get confused sometimes about what formats to buy (when I’m not shopping directly at Amazon, of course!). Here are some great references to help with buying e-books and devices. If you have other recommendations for articles that helped, let me know!
E-Book Format and Device Table
Comparison Table of Reading Devices
Reading e-books without a dedicated device by Tia Nevitt
How to read e-books series by Angela Korra’ti