This is the freebie book when you go on the RYA "Level 1 - Start Sailing" course. You can tell how far beyind I am on book reviews, since I went on the course on 13-14 April!
Let's ignore the book for a moment, and focus on the sailing. I'd last tried sailing something over 20 years ago, and never got on with it. Cold and wet wasn't my thing, and the physics of sailing towards the wind was not intuitive to me. Fast-forward, and I'm a lot better at ignoring discomfort and non-intuitive physics. I've got to an age where vague exercise in the outdoors where you're not thinking about computers or maths seems a good idea. Being in London, there's plenty of water nearby (including docks, as well as the Thames), so it's pretty convenient.
The book is pretty basic. Sailing, like many other sports and physical activities, is best learnt by tuition and practice. The book covers everything you learn in the course, but more as a reminder than teaching. It'd be foolish trying to sail on your own having only read the book, but it does provide value as it's very easy to forget bits of the course over time if you don't have a reference.
I believe you can buy this book separately from the course. Don't. It's useful if and only if you've been on the course. For those who've been on the course, it's pretty handy.