I just discovered that P. N. Harrison’s 1921 book The problem of the Pastoral Epistles is available for free from the Internet Archive. While plenty of more recent works treat the arguments against Pauline authorship perfectly well, they lack one thing that Harrison’s book has: charts graphing the differences between the Pastorals and other letters attributed to Paul on the one hand, and similarities to many of the earliest church fathers on the other. If you have never read it, it is worth flipping through just for the graphs alone!