Gary - both these books, in my view is about techniques rather than design, Joyce's book being by far the better. I view it as the standard text on 'how to'. A book on how to design furniture is a completely different kettle of worms. Firstly, I would recommend having a look at the numerous books available in woodworking book clubs (GMC do a good and very comprehensive assortment of woodie books).
Secondly, I think that a basic understanding of proportion as applied to furniture is very useful in trying to get to grips with design. If you need to get one book though on design, the essential text is David Pye's 'The Nature and Aesthetics of Design' - Rob