This book is probably the best one for explaining different dress cuts, shape to best suit different people etc. It has plenty of photos in to give you ideas. I bought it to make my own dress but it is just as useful to someone buying a shop bought or commissioning a dress maker to design a dress for you. It will give you the knowledge of the basics so you know the difference between an a-line and a princess cut.
If like me you are making your own dress I would recommend three books. firstly this one for ideas and inspiration and to know which dress will best suit you as well as the variety of different components that make up the dress. After that look at websites/mags for ideas. second is 'Bridal Couture' by Susan Khalje. This is the best I think at explaining techniques with plenty of colour photos explaining exactly how to make each component of dress. It was what I used the most to learn how to sew couture, a must for anyone trying. You won't get any inspiration from the dresses though, it is purely technique. The third if you can get hold of it is 'Vogue Sewing Revised and Updated' this is hard to get at the minute but again it will teach you sewing techniques, even the very basics. With that and 'Bridal Couture' you should have all you'll need to know.