I've just learned about the Tangiers International Zone - and the subject is incredibly fascinating. As far as I understand it, this was, effectively, a free city created by an international impasse?
Does /r/AskHistorians have a book recommendation on the subject? I'm looking for something approachable by a non-professional, but a dense text would work too - assuming it's self-contained.
The only book I know of that is specifically on the topic is "The International City of Tangier" by Graham H. Stuart. It was written in the 1950s for an academic audience, and so while I haven't read it myself, I can imagine it's fairly dense.
For a more 'pop culture' history of the city, "Tangier: From the Romans to The Rolling Stones" by Richard Hamilton may be of interest, but Hamilton teaches the history of the city through a mix of storytelling and biography of notable inhabitants of the city (and it's not specifically about the International Zone), so you're mileage may vary.
"The Law and Practice of International Territorial Administration: Versailles to Iraq and Beyond" by Carsten Stahn has a chapter on the Tangier International Zone, but that may be broader than what you're looking for.
Lastly, for some wider context on Spanish Morocco, "Colonial al-Andalus: Spain and the Making of Modern Moroccan Culture" by Eric Calderwood, and/or "France, Spain, and the Rif" by Walter B. Harris could both be of use. The former is an overview of Spanish Moroccan colonial cultural development, while the latter covers the Rif War, a failed nationalist uprising occurring at the same time that the Internal Zone was established.