Complex sentence: In 1893 he investigated the ruins of Axum and other places in the north of Abyssinia, partially made known before by the researches of Henry Salt and others, and "The Sacred City of the Ethiopians" (1893) gave an account of this expedition.
Split into simpler sentences: In 1893 he investigated the ruins of Axum and other places in northern Ethiopia, which had previously made known in part by the researches of Henry Salt and others. His book "The Sacred City of the Ethiopians" (1893) gives an account of this expedition.