I haven't read that particular book, but having read St. Ignatius, I would guess the biggest blow would be to a low-Protestant understanding of ecclesiastical structure and the Eucharist. With regard to ecclesiastical structure, the low-Protestant view is that for the first couple hundred years of the Church, churches were essentially independent house-churches with little to no hierarchy, until Constantine came around, legalized Christianity, and turned it into a branch of the Roman government. Well, St. Ignatius, writing less than 80 years after the Resurrection, has such choice quotes like,

Take care to do all things in harmony with God, with the bishop presiding in the place of God, and with the presbyters in the place of the council of the apostles, and with the deacons, who are most dear to me, entrusted with the business of Jesus Christ, who was with the Father from the beginning and is at last made manifest — Letter to the Magnesians 2

or:

Wherever the bishop appears, there let the people be; as wherever Jesus Christ is, there is the Catholic Church. It is not lawful to baptize or give communion without the consent of the bishop. On the other hand, whatever has his approval is pleasing to God. Thus, whatever is done will be safe and valid. — Letter to the Smyrnaeans 8

...Which is completely contrary to the low-Protestant view of Church history. And from other writers in the 2nd and 3rd century, it's clear that St. Ignatius' understanding of ecclesiastical structure was not just one man's opinion, but accurately reflected the belief and practice of the early Church.

As for the Eucharist, St. Ignatius wrote,

[The heretics] abstain from the Eucharist and from prayer because they do not confess that the Eucharist is the flesh of our Savior Jesus Christ, flesh which suffered for our sins and which that Father, in his goodness, raised up again. They who deny the gift of God are perishing in their disputes. — Letter to the Smyrnaeans 6

Considering that St. Ignatius was a disciple of the Apostle John (who wrote John chapter 6) and was personally appointed bishop of Antioch by the Apostle Peter, it is quite clear that the low-Protestant understanding of the Eucharist as a memorial...just was not an acceptable belief in the early Church.

Now to clarify, high-Protestants would have no difficulty with this, although I understand some Lutherans, Anglicans, and Methodists have begun to deny Real Presence. But it seems to me that the high-Protestants who deny Real Presence aren't really concerned with what the early Church believed in the first place, whereas low-Protestants at least in theory should be.