I made the mistake of initally buying the book only. However, just a few pages in it becomes clear that the cassettes aren't really optional - they're essential to get the most from this book. Many of the exercises refer directly to the cassettes (even though the book omits to tell you this). I was very confused trying to work out the answers to exercises until it occured to me that the questions referred to spoken passages on the cassettes (which at the time I did not have!).
After buying the cassettes separately I became increasingly confused...
The book uses a 'cassette' icon where ever there is supposed to be relevant audio on the cassette. Unfortunately, it is not consistent. Frequently the icon is missing where there is audio and at other times the audio does not match what is printed in the book. It makes it very hard to follow from book to cassette. I ended up using a highlighting pen to mark the words that do appear on the cassettes.
A Bulgarian friend spotted many other inconsistencies and plain errors in the book and audio, which she was able to correct in the book. If it had not been for her I would've had an even more confusing time.
The book also seems to miss out essential information. In the first chapter it starts asking for answers to questions it hasn't yet taught you how to answer! Even my Bulgarian friend found it confusing!
However, I did learn to read Cyrillic very quickly. After that progress slowed down. I haven't tried learning a language since 'O' Level French more than 17 years ago so I needed a quick refresher on a lot of language terms that are used throughout the book (such as conjunctions, imperative, passive voice, indefinate tenses, past imperfective active participle(!)).
I'm finding it very slow going. Maybe this books teaching methods don't suit my ways of learning, or maybe I'm just not very good at languages. I've ordered another book/cassette combination from Amazon (Bulgarian - Teach Yourself Languages, Michael Holman) to see if I have better luck with that.
However, I'm going to persevere with this book and see if it gets clearer as I continue.