I had to warm up to this book. After over 500, 000 words written in the flowing prose of The Lord of The Rings epic, the appendices were a bit jarring.
Written with very little embellishment, in an academic style, the appendices are nonetheless as beautifully written as such a style allows. These are supplementary stories, most actually quite interesting, that fill in details of some characters and relationships not as fully developed in the main text. It also contains notes on how Tolkien used language and "translation" from the conlangs of his world into our English. This I also found fascinating.
There are also charts of letters and transliterations, maps, and family trees.
Slightly more laborious, yet still interesting, were detailed histories covering all of the third age of Middle Earth. Most interesting were the parts of the timeline that follow the events of The Hobbit, The Lord of The Rings, and the time between the fall of Sauron and the beginning of the fourth age. In these parts Tolkien gives a great deal more detail. I loved following the life of Samwise in the timelines, not included anywhere else in the story. There are also timeline sketches of the other ages.
There is a helpful index also included, pointing the reader to important stories in the main text, an index to which I am likely to frequently refer.
Most tedious were pages of genealogy. As is true for most of this supplementary book, any student of Tolkien would likely find this information an absolute treasure. To this casual reader, it was worse than pages of randomly connected letters.
The Appendices are not essential to understanding The Lord of The Rings, by any means. One could skip them entirely. However, I believe most readers who make it through the entire epic tale will likely enjoy reading the supplementary material, as some who loves a film may enjoy the supplementary material in a Special Edition DVD. Judged this way, The Appendices are of the highest quality.
I would recommend The Appendices to anyone reading The Lord Of The Rings, but would suggest reading them along with the main text, rather than all at once at the end, as I did. It is quite a dense bit of information. Taken in smaller segments, along with the appropriate sections in the main text, would likely be far more enriching an experience.