I read 30 Years A Watchtower Slave back in 1973 when I was still a JW, and an elder. Though the book didn't cause me to leave the JWs at the time, for me it did answer the question as to why Hitler persecuted the JWs in Germany. I never bought the Watchtower's explanation for the persecution. But Schnell's book gave a more satisfying answer. Hitler viewed the JWs as a political threat to his regime, and Schnell's discription of the JW organization in Germany shows why Hitler felt justified in taking extreme action against this American cult.
However, Ray Franz's COC was a much better book. In fact it was reading COC that hastened my departure from the JWs.