I have an iPhone and iPad and have bought books using iBook. I can read them on my 2 Apple devices but I'd like to find out how to read them on my PC.
iBooks are for reading on your iPhone and iPad (iOS devices). They can't be read on a PC or Mac.
See iTunes: About Books
which says in part,
"Books you purchase on a computer must be read with the iBooks app on an iOS device."
"Note: Books are only readable on iOS devices that have a compatible version of the iBooks app installed."
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