The Libreria and the column with St Theodor and his crocodile. View looking Northwest.

The building designed by Jacopo Sansovino (1486-1570), is considered a great example of High Renaissance architecture. It starts with a classical Doric order of columns on the first level and then an Ionic order of columns on the second level. But instead of letting the openings of the building rest atop these main columns -- as would be done classically -- he designs a second set of Doric columns just inside the first and rests the arches of the openings on these -- which give it a more decorative look  He then puts sculptures atop a balustrade rail. 
(E.H. Gombrich, The Story of Art, 14th add. pp.247-248)

Today, the old Library houses the Archaeological museum containing GraecoRoman sculpture and  Roman portraits among other things.

The Piazzetta, Venice and the Libreria
c.1904
 

• More on San Marco
• See Overview of Venice