If you mean iBooks then that is only available for iPads, iPhone and iPod Touches, there isn't a version that works on Macs (or PCs) - and you probably won't be able to read Amazon ebooks in that app. Most ebooks have digital rights management on them which tie them to the store's own apps and ereaders - so Kindle ebooks from Amazon can only be read in their Kindle ereaders and Kindle apps (and ibooks from Apple in Apple's iBooks app).
Amazon have a Mac Kindle app which you can download from here, and there is help for it on this page.
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