As a fellow midiæval geek, I have to say Gregorius Yamada usually is pretty accurate, although some information is simplified.
I did notice
a factual error on pages 5-6: As far as
Western Civilization is concerned, the fact that the world is round was basically common knowledge throughout the Middle Ages (
and earlier)! It was commonly believed that
you would die before reaching any land, however, since they had no indication of other landmasses and
they had a pretty good idea of the size of the earth, with scholars having fairly accurate measurements (a primary source of error was the belief that the Earth's general form is a perfect sphere; we only found out otherwise in the 17th century). On the other hand, Asia was also believed to be even more vast than it actually is, and there were translation problems between sources (e.g. the Arabic mile versus the Roman mile), which often confused things. And there were, as always, naysayers — there
still are — and the ignorant. But generally, most Europeans believed the Earth was spherical and had a decent idea of its scale.