I'm appalled by how easily this punched me in the gut over and over and over again. I've read my share of down-and-out short stories (e.g., anything by Alice Munro). The ones that make you feel pretty good about your own life because if these sad sacks actually exist, you're doing darn well... This was different.
This isn't technically a novel but instead a set of stories woven together by the common thread of this woman, Olive Kitteridge, a schoolteacher in rural Maine. In many of the stories she barely makes an appearance, other than she taught the main character or just lives in the same town. This in no way detracts from the power of the entire tale. I'm not going to go so far as to say that it enhances it, but it is a clever device in that it allows Strout to add more variables instead of needing to use Kitteridge as the one constant.
Each story already showcases someone lonely, down on their luck, or even blatantly crazy. It's too facile to say that what Strout is doing is twisting and bending these stories to make them even sadder. That might be true. What her main intent is, though, is not plot or theme or story. It's reflection. The whole art-is-a-mirror-reflecting-real-life is a tired trope, but when you engage with a piece of art that truly does this, it'll take your breath away. These stories were far more than sad-- they pulled all the bits of yourself that you aren't interested in seeing from your belly to your head so you have to interact with them.
This is not pleasant, but it sure does provide a whole ton of insight into your own life and your past. I wouldn't necessarily say approach with caution because these stories are eye-opening and unforgettable and shouldn't be missed, but be aware of how they may affect you. There's a reason this won the 2009 Pulitzer for fiction.