... :-) ... no jokes please ...
I guess, that one cannot compare coin motifs (which can vary in many directions) and the 6 motifs of the Petrarca Trionfi series (which know some fixed patterns, which at least to some parts repeat their attributes with variants), it are just different fields.
I don't know, why Sforza mixed a horse with a book. And I don't know, if these objects are indeed books. Also there is a sword, not to forget. If these objects are books, they look, as if they are closed by a special installation ...
... like some of these:
But (if it are indeed books) it looks like a very unusual installation to keep the book closed. I would think, that these are not books. Whatever this is ... I've no idea.
Added: However, at this resolution it looks more like books: