Well, he spends a lot of the book just talking about the wind. That can be pretty useful in most coastal situation because it's all about playing the odds. Being able to predict likely changes, understanding the character of the wind is the best basis for forming a strategy. Inland wind is much more chaotic so very difficult to apply his principles. It takes a lot of time to recognise the patterns he describes.
Terminology is poorly explained. Assumes you already are an experienced racer.
The fluid dynamics is poorly explained. It's written assuming you already have some base knowledge.
It's good stuff if you do have that assumed knowledge but if you don't, you need to get it elsewhere.
My 1st aerodynamics book was Martin Simmons Model Aeroplane Aerodynamics (some such title). I was into RC contest glider design at the time. The explanations are simplified yet accurate and well written, certainly better than Bethwaites. However its way more detailed in what it covers. It would contain a lot of information you don't need sailing but about everything you do.
I found Bethwaits book heavy going. Very good content but it could be written better.
You might do better with the "To win" series (Helming to Win, Crewing to Win)
In my opinion, you need books on boat tuning, sailing technique/coordination, race tactics and rules. Then read Bethwaite. You’re not going to do all this in one season.