For me, I find that my Android-capable phone works pretty well as an e-reader. If I have it turned off during the day with the battery removed, and only turn it on at night for an hour or so with it in Airplane mode and the screen brightness turned all the way down, I'll get five to six days' charge out of the battery.
Which, here in the sweltering Southeast, is about the longest I'll have to go between resupply stops. Stopping in town, I'll usually take an hour or two for food and probably coffee at a local place. Which, if I bring the charger, means that I'll have pretty close to a full charge when I leave. Phone & charger together weigh ~217 g/7.7 oz (analog scale, so that's an estimate good to about 0.1 oz either way).
On my bike tours, I'll add an emergency backup battery charger (storage battery; gets me about 50% of full power), since I use the phone as my bike computer/road map. That weighs an additional 67 g/2.3 oz. It's rechargable from the same charger as my phone, and I usually stay at an established campground for about one night in three. Also, I usually stop for lunch at a restaurant with wall sockets.
Stores pretty much as many books as I want, either in .epub format or .pdf format.
Anyway, it's worked pretty well for me so far.