I may love Tolkien’s worlds, but my knowledge isn’t encyclopaedic. I didn’t read this cover to cover — I’m sure some people would, but it’s not the kind of thing I enjoy — but it strikes me as a good encyclopaedia for the world (less a dictionary, I think: it’s not just about the etymology and meaning of words, or even mostly) and a good reference, especially for those who find things like genealogies and far off cities difficult. It’s a well presented book, too: faux-leather, with an embossed cover and nice pages, some illustrations included, and the maps on the endpapers.
Just flicking through it, I’d find myself drawn in and reading an entry or two in whole: the one on dragons spans several pages, for example. It covers a lot of the more obscure stuff, from The Silmarillion and beyond; I’m not sure how much it draws on Tolkien’s unpublished papers, given the difficulty of figuring out what is meant to be canonical. I’ll update this if I ever find out definitively.