The Plural of Book
I just finished a book called The Unfolding of Language by Guy Deutscher. It's pretty awesome if you're interested in a) how grammar evolves over time and b) how the very notion of grammar (verb tenses, prepositions, etc.) emerges in the first place.
The author describes, for example, the Latin case system, where nouns can have different endings depending on what role the noun is playing in the sentence. You say "cactus", for example, if it's used as a subject, but you use "cactum" if it's used as an object.