This book has a very specialized appeal, to "conlangers" -- people who make up languages as a hobby. The secondary audience would be writers who want to make up a language for a science fiction or fantasy setting. Even if you don't want to create an entire language, like Tolkien's Elvish, this book gives plenty of tips for making your bits of vocabulary and phrases believable.
It also serves as a decent linguistics primer. Rosenfelder covers the basics of phonology, morphology, syntax, orthography, and other linguistic topics. Anyone who has studied linguistics will find this material very introductory, and if you just want to know how to make up a few believable words for your fantasy language, you may skip that part, but it's helpful for those who really want to get into constructing their own languages.
Despite being textbook-like in nature, the LCK is not dry. It contains many linguistic anecdotes, examples from both real and fictional languages of language features of interest, and discussions of common mistakes people make when inventing languages.
As a very specialized toolkit, this is a densely-packed book full of useful stuff. If you have any interest in constructed languages and are wondering whether to buy this book, do. However, if your interest is more in general linguistics, there are better introductory texts out there (though the bibliography in this book is also quite thorough).