The title of this book is a bit misleading. It does not deal in great detail with Hancock's actions at Gettysburg and his impact on the battle. Instead the writer centres on the Howard-Hancock 'controversy' and the Hunt-Hancock controversy.
The first is only marginally interesting because it had no real impact on the battle and only became a thing in post-Gettysburg writing. The second one might have had far reaching effect on the outcome of the battle.
I would have hoped the writer had gone more in depth on Hancock's actions and their consequences. I did not get a good appreciation of his crucial part in the great contest from this work.
Still it is well written, albeit a bit tedious at moments, and has an interesting level of detail in some respects if this is the hundredth book on the battle of Gettysburg you are reading. Otherwise, stay clear for now and first read books like those of Pfanz etc.