This book's subtitle, "A brief handbook and website for Schenkerian analysis", is a bit too modest. Although short, it's a very complete and thorough first course in Schenkerian analysis.
One good feature of the book is that Pankhurst describes Schenkerian notation at the same time as introducing the concepts. This means that he needs far fewer words than other writers to say what needs to be said. The examples often speak for themselves, and everything is very clear.
Another excellent feature is the section about how to do a Schenkerian analysis and how to present one in practice. I don't know where else one might find this. The exercises at the end are well graded and there are a few very easy ones on the web site to get you started.
Schenker's theories are controversial and many people find them quite hard to swallow. Some authors either skate over this or else patronise and bully their readers. Here's what Pankhurst says: "Whereas Schenker lived at a time when knowledge tended to be presented as absolute truth, we tend today to view it as somewhat more provisional. The task for a student of Schenker is to be open to understanding music in the way that he suggests, but at the same time keeping critical faculties intact and alert." He returns to this issue several times during the book, always with the same undogmatic good humour.
Normally you'd want to be at least at undergraduate level to tackle Schenker's theories, but this book is so clearly written that I think a good A level student could manage it pretty well. I wish all music theory books were written as well as this.