What? At what point does forced conversion become significant to you? Is there a threshold of people being killed for not adopting the new religion beyond which you'll care? What is it? 5,000? 10,000? 50,000?
You really need to learn critical evaluation of sources - that book would have been written by a Christian source - the one thing you can say for Christianity is that it spread literacy across Europe - and it would have been in their interest to malign local beliefs in order to make their own seem more true and just. Maybe it was true, maybe it wasn't - without a more objective source we'll never know.
Then you're ignoring the example of history. In most places where Christianity showed up the missionaries and monks worked their way into the retinue of kings and local chieftains who they could then convince to order their people to the Christian faith. And the top-down model of conversion worked - it allowed Christians to destroy Pagan sacred sites - or co-opt them entirely - and build churches on them. Okay, sure, they converted peasants and lower-class people at first but their ultimate objective would be the ruling elite. The Saxons are a good example of this - Charlemagne converted the ruling class but the rebellions the Holy Roman Empire faced came mostly from the ruled people who clung on to their worship of the Old Gods. Then when these people decided they didn't want to be ruled by an elite who wouldn't let them practice their beliefs they were slaughtered.
The subject of forced conversion was so important to the spread of Christianity that it was even touched on by Pope Innocent III in 1201. From Wikipedia:
"
Pope Innocent III pronounced in 1201 that even if torture and intimidation had been employed in receiving the sacrament, one nevertheless:
...does receive the impress of Christianity and may be forced to observe the Christian Faith as one who expressed a conditional willingness though, absolutely speaking, he was unwilling. ... [For] the grace of Baptism had been received, and they had been anointed with the sacred oil, and had participated in the body of the Lord, they might properly be forced to hold to the faith which they had accepted perforce, lest the name of the Lord be blasphemed, and lest they hold in contempt and consider vile the faith they had joined."