The Lord of the Rings: The War of the Rohirrim was one of the most unexpected announcements for the Tolkien fandom. Whether or not we're wary of an Amazon Lord of the Rings tv show, we're more or less used to the idea by now, although a Lord of the Ringsanime might seem even stranger to some. Nevertheless, many fans were excited to hear the announcement and might be wondering if Lord of the Rings: The War of the Rohirrim is based on a book by J. R. R. Tolkien.
First, a little background: if you need to refresh your memory, the Rohirrim are the people of Rohan, one of the most well-developed locations in Middle Earth. In the Lord of the Rings book and film trilogy, Rohan is ruled by Theoden, while Eowyn, a fan-favorite character, is also from Rohan. The Rohirrim participated in battles that took place during the events of The Lord of the Rings but the titular war, which is going to concern the new anime, doesn't happen in the main story.
What is it based on, then? It's no secret that only The Lord of the Rings and The Hobbit can be adapted for the screen, due to a deal made when Tolkien was still alive. On the contrary, those works that were edited by his son, Christopher Tolkien, and published posthumously cannot be adapted, as the Tolkien Estate retains the rights for those.
It turns out that the events of Lord of the Rings: the War of the Rohirrim unfold 250 years before the story you know and love. The anime will tell the story of Helm Hammerhand, a legendary king of Rohan. If the name doesn't ring a bell, Helm's Deep probably does: it's a location you saw in Lord of the Rings: The Two Towers.
As for Helm's story, this can be found in the appendices at the end of the Lord of the Rings book. So, strictly speaking, The War of the Rohirrim will actually be based on The Lord of the Rings even if this doesn't initially appear so. Of course, the story might be significantly expanded, but now you know where to find it, so why not give it a read before the anime comes out?