The first book that immediately popped into my head literally made me cry, and that almost never happens when I’m reading, no matter how heart-wrenching the content. The book in question was <i>Challenger’s Hope</i>, which is the second book of David Feintuch’s Seafort saga — a space opera series which is, essentially, “Horatio Hornblower in Space.” (So yeah, it was right up my alley.)
I was not expecting to be brought low by a rollicking space-navy adventure — but in the second book of the seven book series, (SPOILERS AHEAD, OBVIOUSLY — so stop reading if you care in the slightest), the main character’s wife miscarries, and is so grief-stricken that she kills herself. She throws herself out of the airlock because, being on a ship, of course, they had a funeral for the child where they had jettisoned the body, and in her note she says she hears the baby crying, he’s cold so she’s going to bring him a blanket.
Of course, this is because at the point I read this, I had a baby in the house, and I was doing that father-of-a-baby thing and CONSTANTLY checking on him. So yeah, it hit me HARD.
So there ya go — scarred forever by Hornblower In Space, to the point where it leaps immediately to mind for this question.