What is like a book, but isn't a book

You already provided your answer. You want a single purpose device. As close to being a book as possible, with none of the extra bells and whistles that are so pervasive in e-devices today. There's no substitute for the feel of a real book. Turning the pages and being able to flip back and forth at will. It's always there and available, with no battery packs to die or connections to worry about.

Of course the Nook and the Kindle are the closest you'll come to simplicity of use, single purpose built, with the benefits that technology is supposed to bring to modern devices. It's wonderful to have dozens of books available in memory or an SD card. And, you won't need to build a library to keep all of them in. To me, that's the advantage of reading with an electronic device; convenience, ease of use, and portability.

The specific model to choose is a matter of personal taste, depth of pocket, and the type of reading you plan on. If you will be reading books with illustrations you will probably want a color model, like the Kindle Fire. I strongly suggest a high definition model, whichever way to decide to go. High definition (HD) will improve the readability of everything, including plain text. The rest all depends on your budget and taste.