The book Extraordinario Libro is definitely extraordinary compared to the other books Sebastiano Serlio has written. The whole Extraordinario Libro is dedicated to portals, ranging from rather subtle entrances to private houses to huge, monumental portals leading into important public buildings. He clearly prioritizes the illustration of portals rather than the description of them: After the introduction, he shortly describes the different portals in 3-4 sentences in the list of contents (which is very short taking the complexity of the portals into account), but after the short descriptions, a whole page is dedicated to each previously described portal, displaying a detailed illustration. The illustrations are drawn two dimensionally: One looks directly onto the façade of the portals.
The book seems to be an academic study of portals. It seems like Sebastiano Serlio was experimenting with multiple orders and combinations while designing his portals, sometimes leading to very unique (and even a little bizarre) settings. His study consists of 30 rustic and 20 delicate variations.
This especially makes the Extraordinario Libro a unique book: The 5 books that Sebastiano Serlio had published previously were written in a handbook-like way, where he replicated and explained the perfect ancient orders that Vitruvius had established. They give a clear implication of what good and bad architecture is and they don`t create a lot of space where an architect can be creative. Therefore it is striking that Sebastiano Serlio mixes and matches so many different ornaments and orders, apparently neglecting the rules he himself had established. He later justified his approach, stating that his examples have to be reduced to a solution which is more in accordance with the ancient Vitruvian orders.