The "photomosaic" is used usualy for big works with big scale,at least for poster like for a movie,maybe you You have seen "The Truman Show" movie,the poster of the movie was in this way/style,and I love this poster,too!
But,for a cover of a book,using a lot of mini pictures with a title,TE logo,...with notice to scale and size of a book,will make an eye confusing for viewer at first time that they see the book.
Most of designers don't use photomosaic for a book usually.
cheers!