The Pleroma An Essay on the Origin of Christianity
by Dr. Paul Carus
One of the finest, briefest histories of Western religious thought we have been fortunate to find. He comments on both John The Baptist and St. John of Revelations. To the best of our knowledge, this is not a book Joel mentions, but it may be one he read as it was published in 1909 and Carus was a brilliant, deep, liberal thinker who may have interested Joel.
If you have heard Joel's mention of the future unfoldments which will give us the original teaching of Jesus and have wondered what they could be, please look to the Nag Hammadi discovery in Egypt. The National Geographic Society has recently released studies on The Gospel of Thomas and the Gospel of Judas. Pursuing these, you will come to a beautiful discovery of your own. Here are some references and sites if you are interested:
The Gnostic Discoveries by Meyer
The Gospel of Judas by Kasser, Meyer, Wurst
The Gospel of Thomas by Meyer
Please continue your own further study. It is a fascinating and beautiful development in Christian teaching made sweeter by Joel's foretelling.