One of my profs is working on an electronic casebook (with West or Lexis, not sure which). He talked about it and from what I understand, you get a hard copy of the book and book comes with some sort of access key to a secure site where the electronic version is located. The electronic version will have links to cited cases, statutes, terms of art, whatever inside of it.
I asked him if you could take notes in the electronic casebook and he said he didn't think that was how it would work. You could copy and paste the text into another program like a word processor or note taking program, and type in notes from there.
I could see how it might be really useful with OneNote since I think OneNote lets you tag portions of text (e.g. maybe you could create a tag for "holding", "rule", etc. and apply those to the case and also type your notes alongside the text. I am not a heavy OneNote user though so I am not entirely sure I am being accurate as to the functionality.
I'm really curious about it at any rate. I had my books unbound and rebound into smaller pieces because of the weight. Works really well, but it would be even better to not need to carry them around at all!