I bought the trilogy as 3 books in one. And that is how you should read them, as 1 book. The 3 books are the 3 main chapters in the life of the main character, Tora.
There is so much pain in the book(s), so much suffering and loneliness your heart will sometimes break, but together with the maincharacter you will fight back, still too many pages to go/read, you can't give up, you have to go on and hope there is a light at the end of the tunnel.
So I suggest if you decide to read the story of Tora,( how she fights back, being a bastard child of a german soldier and norwegian islander, how she tries to connect with her mother, how she tries to avoid her stepfather of coming in her room at night, how she copes with an unwanted pregnancy, ... ) you buy the 3 books or the omnibus ... because when you start reading about Tora, you won't be able to let her go. You'll become her, and she'll become you.

Wassmo's spare and lyric prose is a joy to read. In unabashed language she tells the story of Tora, a young girl growing up on a remote Norwegian island a decade after World War II. The illegitimate daughter of a German soldier, Tora lives in a poor household with her drudging mother and a brutal step-father who subjects her to repeated sexual abuse. Fiercely silent, Tora deals with her ordeal alone, groping blindly for safety and an understanding of her predicament--and, at last, she becomes strong enough to overcome it. Her tale is set in the context of life in a small island village where the economy is based on fishing and small everyday dramas constantly unfold. This is the first volume in a trilogy about Tora; unfortunately, the other books in the series seem not to be available yet in English translation. I, for one, having also read "Dina's Book" with immense admiration for the author's talent and skill, will be waiting impatiently for them. Wassmo is a splendid writer and "The House with the Blind Glass Windows" is an excellent way to make her acquaintance.