Arch of Septimius Severus
The reliefs on the arch of Septimius Severus show victories over the Parthians. The men who carved this arch, unlike those who carved the Arch of Titus, used stone drills to cut away the stone, creating areas of high contrast darkness and light.
Over the carvings there was an inscription with gleaming bronze letters, but the bronze has been stolen away, and only the nail holes and grooves for the letters are still there. That's enough for us to read the inscription, and also for us to see where Septimius Severus' son Caracalla, when he became emperor and killed his brother Geta, had his brother's name scratched out of the inscription. Can you see it on the third line from the bottom?
The Colosseum & the Roman Forum, by Martyn Whittock (2002). Easy reading.
The Roman Forum, by Michael Grant (1970). Out of date, but Michael Grant is an entertaining writer with a simple style which teenagers may appreciate.
Ancient Roman Art, by Susie Hodge (1998). Easy reading.
Roman Art: Romulus to Constantine, by Nancy and Andrew Ramage (4th Edition 2004). The standard textbook.
The Arch of Septimius Severus in the Roman Forum, by R. Brilliant (1987). Unfortunately out of print, but libraries should still be able to get it.