There is much more to explore in the area of Old Testament and Inter-Testamental precedents for Trinitarian theology. Below are some scholarly and “layman” resources that will help you to do your own research and come to your own conclusions.
The Unseen Realm by Dr Michael Heiser is available in physical and digital formats. This book is important for reminding Christians of the supernatural aspects of their worldview. Aside from Trinitarian material in the Old Testament, it also discusses the ANE (Ancient Near East) context of the Bible, and the role of spiritual beings in the war against God and the Gospel.
Michael Heiser’s website (http://drmsh.com) has a cornucopia of articles discussing Biblical interpretation and other esoteric matters. He frequently reviews academic material and where possible provides access to articles so that you can read them for yourself. He also creates commentary podcasts (he is currently working through the Book of Ezekiel) and uploads lectures and short videos to his YouTube channels (https://www.youtube.com/user/professorheiser and https://www.youtube.com/channel/UC0Ud2F59K45tT5dujVD0Qzw ).
Alan Segal’s Two Powers in Heaven: Early Rabbinic Reports about Christianity and Gnosticism (Leiden: E. J. Brill, 1977) is a seminal academic work that discusses the changes in Judaism from the “two Yahwehs” view discussed in this series to the “strict monotheism” of later Rabbinic Judaism. Michael Heiser discusses this work in this video: https://www.youtube.com/watch?v=CUkhWBKCuXc&feature=youtu.be .
The Anchor Bible series is also a valuable resource, and if you can access copies of the individual commentaries, then it is worth consulting the discussions of passages concerning theophanies and the “Angel of the LORD”.
C. O’Neill’s Who Did Jesus Think He Was? (published by Brill) contains a chapter on Jewish Trinitarian and incarnational ideas during the Second Temple period, which can be read on GoogleBooks or in this PDF: http://www.michaelsheiser.com/TheNakedBible/ONeill%20Trinity%20and%20Incarnation%20as%20Jewish%20Doctrines%20GoogleBooks.pdf