This was an interesting and emotional read.
It is told by Adam, who has an unspecified but severe learning disability. One of his routines is to watch from windows - hence the title.
It is mostly set in his mid to late teens, as he copes with his family moving to the country, he and his siblings going through teenage stresses, and the inevitable parting of ways. It is a little slow in the middle, but picks up strongly and shockingly towards the end.
In "real" life, Adam has a vocabulary limited to a dozen or so "words" that approximate to English equivalents, but in his head, and narration, he is a compelling mix of insightful and naive, endearing and infuriating, brave and scared.
Aspects of those contrasts are problematic, though. Adam is so eloquent and perceptive in his narration, can remember events from years ago, imagine hypothetical situations and has a theory of mind. This is necessary for him to tell his story, yet it is somewhat undermined when he repeatedly refers to not understanding the words other people use - even those he has just quoted and pondered. If you can filter that conflict out and focus on his problems and frustrations from trying to make himself understood, you'll enjoy the book better.
Another reading tip: the sibling sequence is , starting with the oldest, Adam, Jake, Joss, with about one and a half years between each. I didn't work that out till near the end, which is probably the author's intention, but there were times when I felt I needed to know their approximate ages so I could work out how their behaviour compared with what is typical.
Full disclosure: I read this at the request of the author (I hope that admission doesn't invite a flood of such requests, as I don't usually take them up). Although he sent me a free PDF version, I did actually buy a paper copy, so I feel more able to give the honest review he asked for.