I've always used folio style cases (the kind with the cover that folds back when you open it) on all my e-readers and my 7" tablets, so that's what I bought for my HD+. Based on reviews and price, this is the one I chose:
http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1 . It's well-made and I like the color. However, I found that it felt heavy and awkward and really detracted from what was otherwise a fairly light-weight, comfortable to hold device.
My solution was to buy a silicone case and a zippered sleeve. It's much more comfortable to hold and use. These are the ones I chose:
1. Silicone case:
http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1
2. Zippered sleeve:
http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1 . This particular sleeve is no longer available from Amazon but there are others that are similar.