Regarding The Finished Mystery WT book and its writer and why it has so many bizarre teachings the "Then Is Finished The Mystery Of God" WT book (copyright 1969) makes the following interesting claim (and excuse) on page 252:
"It is well known that a frequent commentator on the book of Revelation, namely, the first president of the Watch Tower Bible & Tract Society, Charles Taze Russell, was looked to by many to write his explanation of the complete book of Revelation. But before he could do so, he died two years after the end of the Gentile Times, or on October 31, 1916. The following year, in July of 1917, the Society under its second president, Joseph F. Rutherford, published the book The Finished Mystery, which set out a commentary on the whole of Revelation, as largely based on the previous writings of C. T. Russell.
In course of time The Finished Mystery proved to be unsatisfactory, because it had been written and published before many critical parts of the book of Revelation were fulfilled to make possible a correct understanding. So, although an attempt was made to write down things heard, The Finished Mystery did not break the secret of any "seven thunders" that had "uttered their own voices." (Revelation 10: 3) It was therefore a time to wait for accurate understanding to be made possible, and in the meanwhile to absorb spiritual knowledge from the hand of Jehovah God by means of his glorified Son Jesus Christ. This was done."
But, the "Then Is Finished The Mystery Of God" book has its own problems, though to me it is far less problematic (and far less outrageous) than The Finished Mystery book. The "Then Is Finished The Mystery Of God" book can be read at http://www.strictlygenteel.co.uk/mystery/mysterytitles.html . Today I have been comparing the interpretations/commentary of the two WT books with each other and I find a number of the differences to be rather shocking.
I see virtually no value in the content of the The Finished Mystery book other than to show open minded JWs (JWs who are open to the idea that the JW religion might be a false religion) and those considering becoming JWs, that Jehovah God and/or Jesus Christ did not choose the WT religion and/or the WT society in the year 1918, 1919, or in the years 1920 - 1929.
By the way, "Then Is Finished The Mystery Of God" book encourages it readers to read really old WT articles and old books authored by Rutherford. For example, page 254 has the following footnotes:
'* For an interesting comment on Revelation 10:6 according to the Authorized (King James) Version see The Watch Tower as of August 15, 1925, page 247, paragraph 33. Also, the book Light, Volume One, pages 178-180, under "Time No Longer," published in 1930.
† For an interesting suggestion on this see The Watch Tower as of April 15, 1919, pages 120, 121, under "The Finished Mystery," particularly the final two paragraphs. A reprint of July 1882.'