why do i need the latest os to read a book bought in itunes?

Reading a book on your computer was never a feature of iBooks, until the latest MAC OS versions.

If you want to read a book on your computer, then you must have the version that allows this. The previous versions did not have this function.

albertsen.erlend wrote:

I am annoyed by the feeling of having to update my OS in order to read a book from itunes store.

any comments, tips, ways around this?

It is a limitation of iBooks that they can only be read on iOS devices or on Macs running Mavericks or newer.

For increased flexibility, get the Kindle app for Mac and acquire books form the Kindle Store. It is available from the Mac Appstore or as a download from amazon.com.