The Return of the King is the 5th and 6th book of The Lord of the Rings epic and its final instalment. In book 5, the members of the fellowship become increasingly embedded in the world of men, as they fight against evil alongside two kingdoms: Gondor and Rohan. It’s almost a medieval military story, with sieges and battles, and a king returning to claim his throne. A few magical elements here and there keep the fantasy aspect alive.
Book 6 follows Frodo and Sam on the last part of their journey to Mount Doom to destroy the ring. Quite a substantial part of the book is about the journey home, which works well as by this stage of the book, you’re thoroughly invested in quite a few of the characters.
One additional thing to note: the appendices at the back of The Return of the King are quite an eye-opener in terms of the scope of Tolkien’s world-building. There are substantial family trees and chronologies, additional historical details and a large section devoted to the languages of Middle-earth, including an alphabet and a pronunciation guide.
Lord of the Rings Books in Order