Nexus 7 supports many formats.
Unlike the Kindle Fire the Nexus 7 does not have built in support for ebooks. Instead you install an ereader application which supports your ebook's format.
Sony has an ebook reader application if you wish to stay with Sony. There are a number of third party applications which also are able to read Sony ebooks.
And if you choose to stray from Sony there are readers for Amazon's ebooks, Barnes and Noble's Nook format and several others.
I think the only ebooks you cannot read on the Nexus 7 are Apple's iBooks. (I have heard of ways to read these but do not know if it really possible or not.)