I'm no linguist, and nowhere near a cunning one (har har). And to be sure, I think this book isn't really about linguistics - at least as I understand the discipline. Instead, it's a series of relatively non-technical essays that look into the structure, history and context of constructed languages.
Far from exclusively focusing on the science fiction/fantasy languages implied in the title, it also includes expository treatment of internet slang, Newspeak (of 1984 fame), modern revival languages with constructed elements such as Hawaiian, Breton, and Hebrew, and early attempts to create utopian universal languages. Heck, the book even takes a stab at looking at the dialogue of Joyce.
The treatment each language receives varies slightly, but in general it couches the structural and phonological descriptions of constructed languages in the history of their development and the way decisions in the constructed languages play out in their speaking communities. Each bit of context is given to help you understand the motivations behind many of the (often idiosyncratic) people that created these languages, or, if no one creator exists, the interactions between the people in charge. The technical descriptions of the languages, by the way, are unusually accessible given the clearly academic origin of some of the writing. I can't remember a single instance of IPA making its way into the text, for example (although there is a short section in 1337). For those who want additional discussion of the languages, every chapter has an appendix, though it may not contain what you want.
I liked this book, but I will easily say that it's not for everyone. The writing style is academic and thus at times very dry, particularly when moving through the histories of early constructed languages in the late 19th century that I'd never heard of. I managed to power through to read the (alas, breathtakingly short) chapters on Elvish and Klingon, but many others may want to skip them and move to the other self-contained chapters.
The book also is more enjoyable in parts if you have the right external context. Two chapters in this book stand out to me in particular. One covers Newspeak and Nadsat, two constructed languages/jargons from 1984 by George Orwell and A Clockwork Orange by Anthony Burgess. The other covers "Oirish," the attempt to differentiate Irish English as a written language from Standard English, as written by James Joyce in his many novels - including, at the end, a discussion of Finnegans Wake. Reading about the way language is used in books you've never read is sometimes interesting, but you will get more out of it if you've read those books. Given that I have a realistic idea of how many people have attempted to read (let alone completed) Finnegans Wake, I would advise the average reader to just skip that chapter.