The book that splits the party, and the structure is the risk. Two separate narratives running in sequence rather than interleaved, so you leave one set of characters for a few hundred pages at a time.
Helm’s Deep gets the attention. The better half is the other one, where the going is slow and miserable and Gollum is doing most of the real work.
Read April 1988, days after Fellowship. It took me until the following summer to finish with The Return of the King.