In The Great Angel. A Study of Israel's Second God (London: SPCK, 1992) she tested the hypothesis that when the early Christians read the Old Testament as an account of the pre-incarnate Christ, they were reading in a traditional way and were not innovators. She proposed that pre-Christian Judaism was not monotheistic in the generally accepted sense of that word. From a comparison of ancient versions of the OT she proposed that Israel had known a High God and a second, national God, known as the Son of God Most High. Since crucial textual variants arose relatively late, as can be seen from the Qumran evidence, the second God remained a living issue during the second temple period. The hypothesis was tested in Philo, early rabbininc texts (building on the work of A Segal Two Powers in Heaven Leiden: Brill, 1978, but reaching very different conclusions), in Gnostic texts and, with unexpected success, in the Christian writings of the first three centuries. Finally she tested the hypothesis in the New Testament where the results convinced her that this was the key to understanding Christian origins. She concluded that when the Christians declared "Jesus is the Lord" they were affirming that Jesus was the final manifestation of Yahweh, the national God of Israel in the Old Testament. Thus the origins of Trinitarian belief are pre-Christian, and the heir to temple tradition is Christianity. The sensitive nature of these results made further study imperative, but nothing she has discovered since has in any way altered these conclusions. The Lord of the Old Testament as the Lord of the New Testament was fundamental to all her subsequent work. This book caught the attention of Mormon scholars [Latter-day Saints] who now take a great interest in Margaret Barker's work.I think you'll also see some strong LDS themes in her 2003 book, The Great High Priest. Interesting!
A valuable LDS resource dealing with some of Barker's ideas is Kevin Christensen's work, Paradigms Regained: A Survey of Margaret Barker's Scholarship and Its Significance for Mormon Studies, FARMS Occasional Papers (Provo: FARMS, 2001).