This is a light novel written by Tatsuki Saitoh, based on NATARIE IN THE DREAM (NataP)'s song "太陽を知らない人" (Taiyou wo Shiranai Hito, officially translated as "the man who knows the sun never", but that sounds a tad Engrishy to me)

It features three Vocaloids NataP released solo albums for: Gakupo Kamui, Luka Megurine and Len Kagamine.

Some links of interest:

Nocturne, NataP's second album, featuring Gakupo Kamui songs (backed by Luka Megurine)

Rakuten page for the novel (used)

How about we start with the song that inspired the novel?

 森よ開け

The Man Who Doesn’t Know the Sun

In a forest deeper than the seas

A man who doesn’t know the Sun

No matter how hard he looks upwards

Only taciturn trees cast their shadows

Even if winter ends and spring comes

Even if summer withers and autumn blows

Inside the forest where seasons don’t reach

He just dreams of something unknown

O forest, open 

Swayed by the wind

The blue of the heavens seen in dreams

A color you have yet to know

In a forest deeper than the seas

A man who doesn’t know the Sun

No matter how hard he looks upwards

Only taciturn trees cast their shadows