Three Musketeers by Alexandre Dumas (Dumas got paid by the line, which had an interesting effect on the work, mainly giving it a modern readability due to the dialogue-heavy scenes and comedic situations the characters constantly found themselves in)

Northanger Abbey by Jane Austen (the first written of her novels, it is definitely the most like YA, but in a definitely good way. The characters are relatable (you may not ever meet a Lady Catherine from Pride and Prejudice, but everyone knows a John Thorpe in their lives) and the satire is relatable. Just change out gothic romances for YA or crime thrillers, gigs for sport cars, and Bath for London or New York and the satire still holds up)

Don Quixote by Cervantes (absolutely hilarious, with scenes that feel straight out of Monty Python or a Terry Pratchett novel)