So, having realised that it was in fact my computer that was hideously slow and temperamental, rather than the Nook or the library software, and, having now acquired a new computer, I have been downloading books from the library onto our three Nooks as often as the library rules allow.
I have a niggling doubt that downloading books to an electronic device cannot be quite as good for libraries as actually going in there, but since we do that too pretty regularly, it probably doesn't make too many odds for us.
The kids have enjoyed All The Best by Roger McGough on theirs, and we are currently reading The Baker Street Boys: The Case of the Limehouse Laundry at bedtime. I only picked it because we sometimes get the train from Limehouse station to see friends in Southend, so I thought the kids might relate to the setting, but it turns out it's a cracking tale so far, and it's one of a very long series, all of which are available through the library service. Bonus!
I very much enjoyed The Comfort of Things by Daniel Miller - all about the residents of a South London street, and their possessions. This is a book I probably would never have picked up in reality, or bought on the kindle, but it was a very interesting read. I did suffer the disappointment of downloading a book which I thought I would love, only to find that I'd actually already read it on holiday a couple of years ago (it was The Gallows Curse by Karen Maitland, and I did enjoy it)
The Nook Simple Touch appears to be out of stock at the moment, but I will update when they come back in - we have been really very impressed with ours, and being able to support libraries and get books for free has to be a bonus. Even if you do download the odd book you've read before.