The question in the OP did not ask whether or not Mr. Davies had some good historical analysis. The question is whether, in 2014, it is “safe” (spiritually beneficial) to read Davies. Keep in mind the secular culture, and large parts of Catholic “Left” and “Right” are far more hostile to the authority of CURRENT popes and bishops than when he was writing. Also keep in mind Davies’ books are promoted by an organization hostile to the current pope and bishops.
There are other books, including some available through Catholic Answers ,that would offer good liturgical historical analysis, but able to incorporate insights that Davies did have access to. Davies’ critique of the OF is somewhat outdated, since the new translation about 16 months ago.
Davies’ books have been identified by some leaving the Catholic Church as the cause. Has anyone identified his books as the cause for anyone entering or returning to the Catholic Church?