Rosenfelder does it again. This man with his admittedly niche hobby of creating languages for his fictional worlds, has produced a superlative volume explaining, in great detail, how languages are put together, how they work and how they evolve over time. All elements, from the creation and modification of the sounds of languages, syntax, grammar and vernacular usages are all covered in great depth. Much of the stuff covered seems so obvious that you wonder why he covers it, until you discover that language features integral, even fundamental, to the use of English are entirely lacking in other languages.
If you have any interest in how languages work, evolve or are created, or if you want to populate a novel or a map with names and places which have more consistency than the semi-random letter clusters many authors use, you simply cannot do without this excellent book. I cannot recommend it highly enough.