I spend quite a bit of time thinking about the English sound and spelling systems, for...familial reasons. I went looking in Wikipedia's footnotes for an accessible account of Wells's Lexical Sets, and landed on this open-access textbook (ecampusontario.pressbooks.pub/lexicalsets/) which I then read with pleasure. It's a training manual for actors working on English accents, but the explanations of the vowels were so clear, and the information about various accents and their relationships so entertaining, that I simply read it for pleasure. Armstrong works hard to present the material without privileging any accent as a reference, which is a marvelous technique for defamiliarizing the vowel system and revealing some of the historical splits and mergers that I didn't know about in my own accent.