I'm still not sure if my children for example, understands that when I'm staring at an iPad I'm almost always reading a book. Does a vast library in iBooks translate to them as well as the same library on physical books in a bookshelf in the house? My sense is it does not.
And when I'm gone, will anyone find any interest in my iBook library? In its highlights and notes? In the books I've read and re-read dozens of times?
Some of my fondest memories are going over my older or deceased family members' book shelves. I have never, however, gone over anyone's tablet. Part of that is because it's newer, but something about the browse-ability of an e-book misses the mark. I can't see which book is worn from being read and re-read, or brimming with notes and scribbles. In digital, it's hard to tell which books that person found significant, but in physical it's obvious by the condition (or even number of copies) of the book.
It's amusing to read that, for on one side of my family, scribbling in a book would be considered a most heinous crime! I keep any writing to the flyleaf if the book is a gift, but don't otherwise write in them. Another thing that complicates the matter in my family is that we have always been serial second-hand book buyers, and in such a case a book's physical condition is not necessarily an indication of how much it was loved by its immediate previous owner. On the other hand, my grandmother tended to insert relevant newspaper cuttings into the book for the benefit of future readers!
Your grandmother's idea sounds wonderful, though.