This paper (39 page PDF) doesn't talk about the applications of Bitcoin or Ethereum, nor even about the specifics of particular currencies. Instead it looks at blockchain (aka 'distributed ledger') technologies in general, identifies major types of features, and creates a taxonomy using them. Features include, for example, identity management, consensus, or fee and reward systems. As such it's a good overview of the underlying technologies, though it will require some familiarity with blockchain to read. The taxonomy looks good to me and accords generally with what I've seen extant. The circular diagram used as an overview in the paper isn't very readable; it would have been nice had they produced a table.