If you want to read in bright outdoor, get an e-ink display.

Unlike back lit screen, e-ink display "freezes" once the image or text is loaded up. so the screen can be powered down to save power. Display remains even if the screen loses power. When the screen needs updating because you've scrolled up or down, gone to another page, then it will use miniscule of power to update the screen. So e-ink is better if you are reading text or something where you are not having to change the image all that often. E-ink display is not very good if you want to play video games.

How the display gets updated is proprietary and e-ink wont even let chip designers who are building the driver for the e-ink will know. I should know! We hit a brick wall when we were designing a chip for the e-ink display.

Back lit, AM (active matrix) or LCD uses far more power than e-ink display.