Additional resources

Articles

Clojure REPLs: A good overview of the various Clojure REPLs. It features a nice section on nREPL.
Overview of nREPL
nREPL’s hidden TTY transport
nREPL Redux
nREPL: Beyond Clojure
nREPL 0.6
nREPL 0.7: Introducing a Native EDN Transport
Documenting nREPL Middleware APIs
Understanding nREPL Sessions
nREPL 0.7
A Cornucopia of nREPL Updates
nREPL Middleware
nREPL 0.8: Evolving the Protocol

Presentations

The Evolution of the Emacs tooling for Clojure - presented at Clojure/conj 2014; dedicated to the origin and the architecture of CIDER and related tooling. The talk features nREPL prominently.
nREPL Redux - presented at Dutch Clojure Days 2019.

Design

The articles here are quite outdated, but they can give you some historical context about the certain design decisions in the early versions of nREPL.