I haven't read this book so I'm not plugging it.
However the basic thesis that Saul of Tarsus would have met Jesus during His earthly ministry has always seemed to me not only plausible but likely and it is good that a new scholarly work is advancing the idea.
Jesus & Saul were contemporaries (Saul was probably converted within a year of the crucifixion) and as adult Jewish males they would both have been regular attenders at the feasts in Jerusalem.
When we consider that Jesus during His ministry always attracted large crowds to hear His teachings and that Saul as an earnest young rabbi would surely have been intensely curious about Him, we can say with near-certainty that the youthful Pharisee would at some point have been present in the crowd when Jesus spoke. Perhaps he often was.
Nor is this merely a matter of historical curiosity since it helps relieve us of the tired old canard that Paul was unacquainted with the "historical Jesus" and just made up a new religion all of his own.
All the evidence says not.