The Nook tablet is just about the biggest tablet you can slide into a pants pocket and also hold comfortably in your hand to read a book. That is its biggest merit.
At $30 on sale, it is a lot cheaper than a phone, and it can actually phone with Hangouts when near Wi-Fi.
The Kindle Fire 7-inch tablet is bulkier and not as good a buy as the Fire 8-inch tablet.
The Nook tablet comes with the essential Google apps, whereas the Kindle Fires require that you load four files to be able to run the Google apps.
The Nook is slow to navigate but plenty fast enough to read a book.
The Nook also has very limited memory, so that you really need to add a Micro SD card to it to run the basic apps you will need. Configure the card as portable storage.
Here there is a caution. It is too easy to push the SD card over the slot, and lose it inside the device, instead of into the slot.
However, once even a very limited memory card is inserted, it is possible to load apps onto it. Updates need to be disabled. Use Hermit to conserve space.