I think Morgan is looking for a book covering the design basics. His
description of "day sailer/dinghy" means no me just that - a vessel
perhaps for the family to go out in for a jolly on the water. I
certainly do not get any inference that he wishes to design a racing
dinghy.

The John Teale book is an excellent book as an introduction to boat
design and I used it to design a 72ft steel vessel. I also bought two
Dave Gerr books:

The Nature of Boats - excellent on every topic of
power/sail/crusing/racing with loads of useful information in it. A
must if starting from scratch as it will make you pull things apart
afterwards.

Boat Strength - his simplification of all the construction standards
to achieve the correct hull strength in any building material. Again
used in the design of my 72 ft steel vessel to check the hull
scantling strengths.