Introductory books are crucial for our first steps into an academical field. This importance lays a heavy load on these books, so only few of them can shine and light our path through the way we really want. I have many of these introductions (in the field of language and linguistics), yet I enjoyed a few. This book, Understanding Syntax, is somewhere in the middle—it's worth reading, yet it's not great.
The reason you should read this book is it defines lots of fundamental concepts you should know about the syntax so that you could embark a journey in this field. The problem is that it is too concise about it—Tallerman could clearly elaborate more in some parts, but she didn't.
Another point is that the book tries not to include definitions that are given under certain schools of thoughts or by different theories. So, you don't get anything on GB, or functionalism. It tries to stay rather basic, so to speak, about it and only conveys the essentials.
The last point, which bothered me so much when I was reading it, is that Tallerman wants to speak cross-linguistically. So you'll encounter a long list of examples from various languages (some of them you've never heard of). I insist that, for an introduction to syntax, being cross-linguistic is not so much of a help—after all, it's not a book on historical linguistics or typology.
In conclusion, I must say, the book can be a reliable companion for your course of syntax, but not a stand-alone guide for it.