If you want to read novels, you can use the IBOOK developed by Apple, but this program can only read PDF.

For comics, there is a software called Comic Control, which is used by many people. RAR, ZIP compressed package and PDF e-books can all be read. If you read comics, I suggest using this.

In fact, there is no need to jailbreak, jailbreak is very troublesome, and many official Apple services can't stand it.