The book is a wealth of information on the various branches of Ursua. I will tell you more about it as soon as I manage to overcome the language problem.
I am also grateful to the people who helped me get in touch with Isidoro, paricularly Martin Ospitaletche, webmaster of the site on the Valley of Baztan (where the photo of the Ursua Palace is found in one my links) and to Esparanza Ursua Sesma of Navarra, Spain.