I don't have Penton's book before me, but he does provide footnotes, but often the sources are obscure, and difficult to verify.
Timothy White's book is Bible Studentish. He likely got some references to sources from the writings of Paul S. L. Johnson, who wrote many books, including one against the Society that is over 800 pages, and was the most vocal opponent among the Bible Students of Rutherford, and documented the many changes that Rutherford made. White is especially good on the theological issues and debates and splits that occurred. He bases much of his book on original source material rather than secondary sources. As time has gone by, many of these issues are no longer relevant. But he was unaware of the Adventist-Russell connection, which Penton helps to illuminate, but does not go far enough. Joseph Zygmunt wrote a doctoral thesis from a sociological perspective, and remains mostly neutral in tone, and is interested in the changes in the power structure and organization.
Russell is the more interesting figure than Rutherford, is more complex and casts a longer shadow over the JWs/Bible Students. Who he really was remains more of a mystery. The canonization of Russell after his death, shown in the book the Finished Mystery, seriously hampered the movement, neglected the serious mistakes that he had made, and was later rejected by the Society. However, Russell afterwards became a non-figure to the JWs, an unknown.
Although the JWs have written a history book, they lack a true historical perspective on their past. The Adventists do not identify with the period out of which Russell developed, and have little interest in documenting their connection with the JWs.
Oftentimes a movement for reform looks to the past for inspiration, and researches for lost treasures to be revived. Penton's group had a journal called the Bible Examiner, after Storrs' magazine the BIble Examiner, and there were some research done on Storrs around that time, but no lasting reform movement developed out of it. Russell had some mainstream Christian ideals that could be a catalyst for change. But both the present-day Bible Students and the JWs are blind to their history. The Bible Students neglect the fact that many of the problems of the present-day JWs are rooted in Russell himself and his movement, his prophetic speculation, the authoritarianism of the "that servant " doctrine, and the focus on date-setting and the endtimes. On the other hand, many of the positive aspects of Russell were cast aside - the importance of love, democracy in the church, proving all things, freedom and liberty in Christ.
Steve