The books are often split in two parts, due to the
size required by Japanese ideograms. That's why there is a line in the middle of
each "cover", in fact it's a scan of two books set one next to the other to form
the full drawing.
The artist who drew those NJO covers is named
Tsuyoshi Nagano.
For the record, Dark Tide 1 : Onslaught was
called simply Dark Tide in Japan, and Dark Tide 2 : Ruin was
called Invasion of Ithor. Star by Star being quite a large book,
it required to be split in two parts (which individually consist of two parts
each), which are called Star by Star: Predator and Star by Star:
Fallen.