Probably the best of several books of LotR criticism I have read: Kocher is approachable, admiring of Tolkien, and expansively thoughtful about the larger-scale themes the master of linguistics and storytelling as undertaken.

Always cementing his observations with structure, example, and pattern directly from the texts, Kocher demonstrate and moral and cosmic order to Tolkien's universe, reflecting on the roles of its denizens through their own parturition, fates, and philosophies. Is the storyline pre-ordained? Is chance involved or does a "divine" hand guide? What roles for fealty and friendship? What for mercy and environment? How, exactly, do we understand evil in a universe without religion? 

These are just a few of the questions Kocher addresses as he examines each of the realms races, the complexity of Aragorn (a terrific chapter), and the value or lack thereof in presuming simple allegory from Tolkien's lifework. At the work's close, Kocher looks closely at seven other writings of Tolkien to reveal their connections to the central LotR, as well. 

There is a ton of Tolkien writing out there, but Kocher's, written way back in 1972, will stay on my shelf.