Born in Qatar in the early seventh century AD, Isaac of Niniveh (also known as Isaac the Syrian) was the author of a number of very fine writings on the spiritual life which have proved very influential, especially in monastic circles, over the centuries. The first part of his writings was translated into Greek in the ninth century at the monastery of St Saba in Palestine, and thence it found its way into many other languages (including in the twentieth century, Japanese). In 1983 a complete manuscript of the second part, hitherto only partially known, was discovered in the Bodleian Library, Oxford, and chapters IV-XLI of these new texts are edited and translated here for the first time. The remaining chapters I-III, which include four sets of Kephalaia on spiritual knowledge, will be published subsequently in CSCO by P. Bettiolo.