I guess if I have to I will buy the ones I'm interested in on Amazon, but I prefer to check out books from a library rather than purchase them, because otherwise costs adds up fast!
EDIT: Looks like here's a concise list, with some sort of purchasing option, but still not sure about e-format http://www.oup.com/us/catalog/general/series/VeryShortIntrod...
EDIT2: On oup.com, there is a tooltip for "ebook" that says "This title is available as an ebook. To purchase, visit your preferred ebook provider." Looks like purchasing on Amazon is the only way. Shame as they clock in at 6-10 bucks a pop (12 if you don't have prime? not sure why it shows a crossed out 12$ for me), would add up very very quickly if you wanted to tackle a big chunk of the series.
EDIT3: The box sets don't seem to be a deal on Amazon (if you already have prime? Still don't understand prime prices). They are 30.75USD for 5 books that are 6.15USD each. So, it appears to me that the best option to read these books on an e-reader is to purchase them individually on Amazon when you find ones you're interested in.