The War of the Rohirrim: Where can you read about Helm Hammerhand in The Lord of the Rings books?

For the first time in a decade, a new “Lord of the Rings” movie is showing in theaters. The title is “The Lord of the Rings: The War of the Rohirrim,” an animated prequel film that takes place 183 years prior to “The Hobbit.” This film depicts the land of Rohan under the rule of a legendary figure named Helm Hammerhand, a name perhaps familiar if you’ve read J.R.R. Tolkien’s “The Lord of the Rings” or watched Peter Jackson’s films. Helm is the king for whom, the fortress of Helm’s Deep was named.