I really can't make up my mind about this book. I love weather books and sailing, but whether this book is a must for either reader I'm not sure.
I think the Instant Weather Forecasting is a better book for the majority of readers, but both could be immensely improved if the photos of cloud formations always told you 1) where are you looking (north, east, south, west). 2) the direction(s) the clouds are moving 3) the time of day and location. Without that they are just good cloud photos and it takes an effort or guess work to get an idea of the dynamics and changes.
I think that in practice, especially if you don't have the time or patience to digest the data, you will rely on instinct, experience and a feel for wind, location, boat handling, etc. There's more value in gaining experience of your areas wind patterns, asking the locals and above all, being observant and vigilant.