Mark Rosenfelder gives very good advice in this book - it covers everything a language constructer needs.
I mostly bought the book not only for that purpose but also to learn more about the linguistic concepts the book covered. It is extremely useful in this purpose - it explains very complex concepts VERY well, at least to an amateur linguist like myself.
Rosenfelder makes good use of examples, which is key in his explanations. However, I think some of it still could have been done in a bit more detail - some of the more complex grammatical concepts lack examples or examples from real languages. The author should also add more detail into the diversity of the concepts - for instance, with verbs or nouns, he lists an interesting array of possibilities for inflections, but the list is not very extensive. This is especially true of the concepts he explored in less detail. One point in particular I thought would have been well-suited to an expansion was a bit in the section on pragmatics. He mentions by way of an example the extreme difference in the way the Apache language made its speakers think. It was a very good example of just how diverse human language can be from the languages we in the West are used to. More examples of this would help conlangers to open their minds and their ability to see all the possibilities. Another favourite was a point about a language he appears to have invented just for the book, called Eteodäole. His single example had me thinking for days.
He gives a reasonable explanation of phonetics, brilliant sections on grammar, semantics and pragmatics, and his section on his own conlang Kebreni, while somewhat confusing and long, was interesting and helpful. (It is also very good for readers to check out his other languages on his website - as well as the other resources he recommends.) Included was a list of recommended basic lexicon items for conlangers - a useful feature.
I also found the book generally captivating, especially the semantics and pragmatics section. They are both very entertaining sections that broadens our understanding and appreciation of all languages. And it makes you think.
And for people who are constructing languages, you need not look further than this book (or perhaps its recommended resources as well). I'd say that for a slightly practiced conlanger, the book could well assist you in creating a language that is vastly better than many of those used in certain fiction.
[PS: I would love to hear Rosenfelder's take on Na'vi from Avatar - but I can almost predict what he might say!]