Currently Being ModeratedOct 13, 2013 5:30 PM (in response to R Crawford)
It tells you on every purchase page in the iBookstore in the requirements section that an iOS device is required. This will only change when os 10.9 is released later this year.
The only exception would be a book that has no drm protection. Free books are often in that category, plus some publishers like o'reilly. These could be read on a Mac air using an app like calibre.