Robert's book is essentially the in depth book on this exhibition.
The smaller de Guitaut book is a smaller book to accompany the exhibition, it will contain general information and many pictures.
There will not be a complete catalogue, the purpose of this is mainly served by Robert's publication.
An apt comparison of the two books would be from the Victoria & Albert exhibition at the Queen's Gallery two years ago two books were published. The large and comprehensive V&A: Art & Love (which can be compared to Roberts book); and then Passionate Patrons, which was the smaller book aimed at those that wanted a souvenir of the exhibition they had seen without all the in-depth information (which can be compared to the de Guitaut book).
Also the exhibition mircosite should provide a lot of information, the pages they currently have loaded on the Cullinan stones, the Diamond Diadem and Queen Victoria's Diamond Crown all indicate that the other exhibits in the summer exhibition will be explained in detail online.