The Library of Celsus is the iconic image of Ephesus — a two-storey marble façade with three doorways, paired columns, and statues of the four virtues (Wisdom, Knowledge, Intelligence, Excellence) in the lower niches. The original façade collapsed in an earthquake in the 10th century; the current reconstruction, by Austrian and Turkish archaeologists in the 1970s, used the original fallen pieces and is one of the most successful reconstructions on any classical site.
You cannot enter the interior — the building behind the façade has not been rebuilt — but the staircase up to the front and the marble Curetes Street that leads to it are where the postcard photograph comes from. The library stands at the end of the downhill walk through Ephesus and is the natural climax of the visit.