Mom (R.M. de Vienne, Phd) and Uncle B (Dr. B. W. Schulz, FRHistS) wrote three books relevant to Watchtower history: Nelson Barbour: The Millennium's Forgotten Prophet; Separate Identity, vol 1 and vol 2. (Each of them also wrote unrelated middle grade/ young-adult novels. Fun books)
The blog associated with the books is Truthhistory.blogspot.com. It's a marvelous resource though focused primarily on Russell era history.
Not noticed much but important are books by Alan Whitby. He's written Grave Matters (about the Watchtower cemetery); The Ross Libel Trial: The history of a 1913 legal case (includes transcripts); and The Hudson Davidson Debate (1896) (The first known debate involving a Watchtower representative.) His books deserve a wider circulation than they've had.
Harrison's Vision's of Glory is not on my recommend list. It's a personal narrative, and as that it might have some value. Her comments on Russell are "off" historically speaking.