This is the second volume in J. R. R. Tolkien's Lord of the Rings trilogy. It follows The Fellowship of the Ring
and is followed by The Return of the King
In this part of the story, the original fellowship of nine travelers is fragmented. Some seek Mordor and the forlorn hope of destroying the one ring in the volcanic fires that produced it. Others are taken captive, and pursued by would-be rescuers. As the travelers disperse, readers become acquainted with the lands and peoples of Middle Earth. We meet the independent horsemen of Rohan, the foul orcs of Mordor, the proud men of Gondor, and the shades of past oath-breakers, eager for redemption. The schemes of wizards, stewards and wraiths become more clear. The tension builds.
The middle book of the trilogy covers a lot of ground, both geographically and in character development. The characters gather their strength for war with the forces of evil. Tolkien gives his characters distinct strengths which complement the abilities of their companions. There is a growing sense that each will have a part to play in the coming conflict--a unique and indispensable part.
If you have already read volume one of the trilogy, you are going to read volume two. No choice, really.