Hi TailUK,
If you are interested in scale sail then the book by Phillip Vaughan Williams An Introduction to Radio Controlled Scale Sailing Models is a better first choice. The Becker book has some interesting ideas about some specifics.
If your interested in class Racing Yachts then I've heard the Chris Jackson Racing Yachts book is very good. This isn't my thing though I have earlier books by Vic Smeed Model Yachting and Griffin's Model Racing Yacht Construction. My understanding is the Jackson book covers more modern material/techniques. The older books would probably help better if you were interested in vintage class yachts - a lot of their pages are devoted to making wooden boat hulls.
Hope this helps
Duncan