Do you want to get the books from your iPhone TO your Mac? If so, I don't believe there is an iBooks app for Mac OSX... Or even a way to read a book you bought in iBooks on OSX because they have a protection on them.
My guess would be that if you wanted to read a book you would buy an iPad or iPhone... Silly Apple
Well Amazon does have a Kindle app for OSX and iOS, but it still won't read anything that you bought through iBooks.
But if you want to get books from your Mac TO your iPhone you can do this through iTunes. You need to simply navigate through the menu at the top and choose the Add file to Library option and select your books and it should appear. Connect your iPhone and sync your books and it should work fine.