[Quoth]

So no matter how good matt OLED / LCD gets, it can only equal front lit eink, never be quite as good as ambient lit mono eink, which really is like dull slightly grey paper. You can adjust a matt screen OLED or LCD to almost perfectly match a piece of paper or ambient mono eink screen indoors, but as soon as you move, or switch between room and table lamp, etc, the OLED / LCD will look dimmer or brighter, bluer (cooler) or pinker (more rose) and unless auto sensors averaged the page, auto doesn't work. Auto can make it much brighter or dimmer with a small tilt. It only seems to work outside going between sunlit, shadow, overcase. That's what I'd excpect, having implemented auto-brightness on industrial control panels I designed.

If you need the front light on, on mono or colour eink, then a good LCD/OLED can be as good and some with similar battery charge use, if mobile, BT & WiFi is off (none of those needed to read a book). Mobile in standby (in case there is a call or SMS) is lower power use than active WiFi. Video, games, mobile data/voice are the big power users. Local audio playback might be as low or lower power on a phone than ereader if display sleeps and battery might be x5 bigger.