From the keynote description it sounds like the app will support all the same features as found on the iBooks app for iPad and iPhone, and it will access the same 1.8 million title ebookstore.
Early screenshots like the one below indicate that Apple has done away with the faux wood bookshelf found in the iOS version of IBooks and replaced it with a clean white layout.
I don't know about you but I am surprised at Apple's radical about face. Ever since iBooks was launched in early 2010 it has only been available on iOS, and I took as a sign that Apple didn't regard the Macbook as a reading device. This conclusion was reinforced in early 2012 when Apple released iBooks Author, which only ran on OSX.
It seemed that Apple had declared a divide between creating ebooks on OSX and consuming them on iOS, but now it seems that Apple is erasing that divide.
But I wouldn't read too much into it. This could be a sign that Apple is growing more interested in selling ebooks, but I think it is more likely that Apple simply woke up to the fact that they were neglecting to sell ebook content on all of their hardware. Not having an OSX app left money on the table.
On the other hand, I'm not sure how much money was left on the table. Apple also announced today that they've shipped over 600 million iOS devices since the first iPhone launched back in 2007. That dwarfs the 90 million or so OSX devices in use.