After the Forum of Julius Caesar and the Forum of Augustus, later emperors also built more extensions on to the Roman Forum, to make more room for businesses. The emperor Trajan built a new forum about 100 AD.
In back of his forum, built up against the side of a hill, Trajan also built a lot of shops. This was not a new idea - Julius Caesar had shops in his forum too, a hundred and fifty years earlier. But Trajan's architect, Apollodorus, built a spectacular shopping mall, instead of just a row of shops.
Learn by doing: go to your public library
More about Trajan's Market
Roman Architecture, by Frank Sear (1983). The standard college textbook.
The Architecture of the Roman Empire: An Introductory Study, by William MacDonald (1982). Actually not so introductory, but it's got great illustrations that really make the building techniques clear.
Roman Imperial Architecture, by J. B. Ward-Perkins (1992). A more detailed textbook, and harder going.