At present, there are three mainstream reading softwares: stanza, ibooks and Panda Reading.
Stanza supports epub, pdf
Ibooks supports ePub, pdf.
Panda supports txt
But personally, I think pandas are not easy to use and the reading effect is poor.
I prefer to use stanza. I have computer-side software, which can convert txt into epub and other formats, and then transmit it directly through itunes. At the same time, stanza has an online library where you can search and download directly with software.