Reading books in other apps using Voice Reader

Dear all, I've just purchased the Voice Reader app, just to try to get other text to speech voices to read books. In the settings of the voice reader app are a variety of English speaking voices, which are not bad. However, I don't want to use what I regard to be gimmicks for reading messages I could write into the space provided or save the result. What I really want is to have a couple of alternatives to read the books in I Books and Overdrive, which will give the Voice Over English voice a rest from time to time. Is thispossible with the text and pdf books contained in I Books and Overdrive and how do I do it? Your support with an instruction list would be very useful or am I being stupid e.g. perhaps you can't do it or it's just a setting change or double tap on an icon on the app? Many thanks foryour help, Steve.