"That is a beautiful book."
Translation:Dat is een mooi boek.
prachtig is a more accurate word for beautiful, like mooi is accurate for pretty
I agree, mooi is not a high rank while beautiful seems to me to be a very high rank, prachtig would fit better
"Prachtig" is more like "wonderful" or "amazing". Mooi is more commonly used. It's used to define something that is nice or beautiful". De film was mooi -> The movie was nice. Jij bent mooi -> You are pretty.