Leonard Chretien, an ex-Witness who spent 22 years as an official in the Watchtower Bible and Tract Society (the Jehovah's Witnesses), recommended The Finished Mystery because it is an example of the bizarre metamorphosis of Watchtower theology over the last hundred years and is useful in showing Witnesses the problems and contradictions in their religion.
The Finished Mystery was the seventh and final volume in Studies in the Scriptures, a series of books written by the sect's founder, Charles Taze Russell. It is a hodgepodge of false prophecies, rambling discourses on the interpretation of Scripture, and the obligatory rantings against the Catholic Church. The Finished Mystery was printed posthumously in 1917 and was touted as an unanswerable critique of "Christendom."
As the years passed, and as elements of its theology changed, the Watchtower trumpeted a series of bogus prophecies concerning the date of Christ's return. To its embarassment, the Watchtower was unable to reconcile either its new theology or its more recent spate of failed prophecies with Russell's book. In an understandable act of damage control, the Jehovah's Witness leadership withdrew from circulation all volumes of Studies in the Scriptures.
Most Witnesses are unaware of the existence of Russell's books, and for obvious reasons the Watchtower is careful not to allow the rank and file access to them. But you can get a photographically reproduced copy of the book from Witness Inc., an Evangelical apologetics group that focuses on refuting the errors of the Watchtower (503-637-3828).
Two other groups that produce good apologetics materials for use with Jehovah's Witnesses are Free Minds, Inc. and Comments from the Friends.
As with all Evangelical apologetics organizations, however competent they may be in their particular field, there is always the problem of faulty Protestant theology being offered as the "solution" to the errors of the "cults." You need to read around this Protestant bias. The organizations' research is still helpful because of their expertise in documenting the errors and contradictions in Watchtower publications such as Awake! and the Watchtower, as well as in many out-of-print works.