Good display material for your gift book table or rack -- and a book that will attract customers, as one of the finest pieces of bookmaking of the year. The Feodor Rojankovsky illustrations- sharply detailed black and whites, beautifully reproduced, some of them with tint block backgrounds -- full pages, half titles and spots- the good clear type face- the impressive size (13(apple) by 10(apple)) stand out above most of the season's books. But in its subsequent use, it runs into a snag. The size suggests a market at the just- after-picture-book level. The text is definitely Junior High, when the picture book format is studiously avoided. The story traces the conquest of Mexico, from young Cortez, in his first official post- through his years of service to his King, until he won his chance to lead an expedition of exploration, conquest, and religious guidance. A dramatic episode, vividly recounted.