Complex sentence: The author was able to gather first-hand documentation, inaccessible to us today, the earliest phases of the theater in Attica, his work is therefore invaluable for the study of ancient tragedy, even if his testimony is not free from doubt.
Split into simpler sentences: The author was able to gather first-hand documentation from the earliest phases of the theater in Attica, which are inaccessible to us today. His work is therefore invaluable for the study of ancient tragedy, even if his testimony is not free from doubt.