To continue with the theme of books on language, I’d like to look at Latin.
I studied Latin in middle school and high school and I realized immediately that it was a really useful language for any speaker or learner of English or Romance languages (and, of course, it has influenced other tongues as well). Even though I unfortunately can’t read it today, what I remember still helps me, both as a teacher of English and also as a user of the language myself.
But though I learned the language (including all those declensions), I didn’t get a good sense of the culture surrounding it. Tore Janson’s book A Natural History of Latin fills that need. Mr. Janson explains the origins of Latin, how and why it became important, and why it is relevant today. I wish his book had been available when I was in school, because it would have helped me understand Latin in the context of its cultural and historical background.