MoMA launched an iPad app this past week, and the app brings support for the MoMA eBookstore.
You can now buy from a selection of 8 art books with prices between $20 and $50. eBooks purchased on the MoMa website can also be downloaded and read in the new iPad app. In fact, it might be better to buy from the website. The eBooks sold there are DRM free PDFs, which can then be read in your preferred PDF app.
On a related note, this is actually MoMA’s third app and its second one for the iPad. All 3 are free, so you might want to check them out.