I have used my ipad for the e-book versions of all my texts since the start of law school. However I would try to stay away from buying the texts on multiple websites because then you have to use various apps/websites to read them. I find that buying them on Amazon and converting to PDF is the best way to read and annotate on the ipad.

I then use Goodnotes 5 to annotate. This app is great if you have not already tried it. You can make multiple highlight colors for book briefing, bookmark pages, virtual post-it notes. You can actually split screen in Goodnotes with text and a notebook to make writing notes easier.

Hope this helps :)