I will attempt to answer this question, despite not having read the book, so please take my advice with caution.
Looking at the website of the book, zompist.com/syntax.html, you can see an outline of the contents. Rosenfelder describes the book as "a tour of modern syntax".
Being a linguist myself, it looks like the book I'd loved to have had while at university. It covers (English) syntactic theory in a way that looks interesting and not simply dry and theoretical -- he supplements the book by a set of interactive tools to develop your own grammars based on the theoretical ideas of Chomsky and others. He does follow a particular strand of syntactic theory, which is mainly common in the US, not so much in the UK, but he also covers some alternatives to the Chomskyan approach. Rosenfelder's own view of Chomsky is nuanced and somewhat sceptical: he has an essay on this on his website (which I wholeheartedly agree with). Note, however, that this is about English, and these formalisms have been developed by linguists working mostly with English, so if your conlang is very different, they might not easily apply to it.
Apart from Chomskyan Generative Grammar, he also talks about Markov text generators, which would be useful to generate (random) texts in a given language. This might be a useful tool to practically explore your conlang: generate some text, and try to translate it. Several other tools are also aimed at generating sentences, rather than just analysing them. The meaning is often a bit random (as it is not really controlled), but at least the grammar will be correct.
So this is basically a syntax textbook, which will introduce you to syntactic theories while playing around with the formalisms and actively doing syntax (which to me is the best way of learning about them!) If you are interested in grammar, and the formal properties of different types of grammars, this looks like an ideal book. In fact, I have put it on my wishlist with a well-known on-line book retailer.
However, if all you want to do is build a smallish conlang, which might not even have a very complicated grammar, then this book looks like overkill to me. On the site he mentions that there is "a chapter on how you can apply all this to your conlangs". So conlangs are not really the focus of the book, but more a possible application of the described syntactic apparatus to invented languages. I would assume -- from the description on the webpage -- that this is a fairly advanced book, which is great for linguistics students and for serious conlangers who want to make use of sophisticated formalisms when describing their languages. In that sense it will not cover too much of the same ground as his more 'applied'/'practical' books about language creation. If the previous books have kindled an interest in linguistics, though, and you want to know more about approaches to syntactic description, then this might be the book for you.
