The recommended reading link below would be a good place to start.
There are quite a few design books around, though they often get pretty technical. Understanding Boat Design by Ted Brewer gives a good basic introduction. Desirable and Undesirable Characteristics of Offshore Yachts, edited by John Rousmaniere is excellent.
Some sailing handbooks often get into design issues. The Voyager's Handbook by Beth Leonard has an excellent chapter on boat selection. Heavy Weather Sailing by A Cole / P Bruce (I have the 5th ed) has three chapters on design.
Lastly I would just prowl around some of the old postings on this website. Lots of excellent info here.