smartphone always no power during reading

What smartphone do you have currently????

Well, I don't know about the smartphone but I know that there are some battery savers apps over the PlayStore which can help you out is extracting a little more juice out of your battery !!

Most smartphones will have a reading app on the relevant market, though to read comfortably on a phone you'll need a bigger screen, but there's a caveat: the bigger the screen, the bigger demand on the battery. As John says there are battery saving/extending apps which control the connection (3g/4g/wifi), the cpu governor, or the ability to go into deep sleep (where the phone almost hibernates when not being used) but they probably won't help with apps such as kindle for android, moon reader, etc (this is why a kindle e-reader can last weeks on a single charge whereas using a smartphone you'll be lucky to get over 6 hours screen-on time: you don't need a 65 million colour HD backlit screen to read a book).
If you really want a phone that you can read books on then your best bet is probably a device with an amoled screen, as it uses a lot less power than a normal display though the downside being slightly less accurate colour reproduction. There are quite a few models with relatively large screens available so shop around for the best deal.