Without viewing the book, it would be very difficult to provide advice on how to fix.
You could do as Sirtel mentions and find someone to fix, but if you can't afford to do that, maybe upload the book here and members can provide advice or fixes. You can scramble the book if it has copyright, or you don't want it shared.
Use this plugin...
https://www.mobileread.com/forums/sh...d.php?t=267998
It sounds like your css needs tweaking, so it should be a simple fix, unless the book is littered with in-line styling, or some unconventional coding, then it might be a bit more difficult. Won't know until we see it.