I bought this book last year for my Dad as he said he wanted to learn to read sheet music, but having seen it sat on the bookcase unused after 6 months had passed I decided to read it myself. I learnt a little about notation at school but I have to be honest I forgot a lot of this, but this book really helped me to recall the basics and then continue on to learn the more intricate symbols and activities. Now don't be fooled, it's not going to be easy and it's not going to be fully understood in one read through. You will need to be prepared to actually "study" the book, much like a student. You will need to keep notes and also, ideally, need the use of a piano or a keyboard to learn the practical side of reading sheet music.
I personally found it best to learn a little each day rather than read the book from cover to cover in one sitting. The diagrams and descriptions may seem very easy at the beginning but they will start to become more challenging and if you skip chapters you will not be able to fully read sheet music. That's the beauty of keeping notes. I kept going over things I didn't understand again and again until I had fully understood the lesson. Sometimes I would leave it overnight and continue from the same point the next day in sheer determination to understand all that I needed to.
Basically If you really want to learn then this book will definitely help you on your way. It's not the end of learning sheet music but the beginning so always keep your eyes open for other books on this subject.