To find it, go to Tünel Square (at the upper terminus of the Tünel underground train). Stand in front of the Tünel station, walk straight ahead into the TünelPasajı, past the KV Restaurant-Café and, out the far side of the passage, the street ahead of you is Sofyalı Sokak. Eren Kitabevi is a few steps along the street on the left-hand side.
You can also get to Sofyalı Sokak by turning off İstiklal Caddesi onto Asmalımescit Caddesi, then left onto Sofyalı Sokak.
—by Tom Brosnahan
Good selection of English-language titles at Eren, on interesting Sofyalı Sokak.