What is the best framework for developing application for ebooks?

Question

Hi,

I'm working on eBook application that can work on different devices(eReaders) and formats.

What is the best framework(.NET, XCODE-apple-,...) OR format(pdf, epub, mobi, plain text, azw) OR devices (iPad, Kindel, Sony, Kobo,...)

the idea is creating application that can change eBook text style (bold, italc, ...).

thanks