A shelf for the child who has gone quiet and flat and can't tell you why. Not the child grieving something specific — the one who's just low, who says nothing's wrong and means it, who used to want things and now shrugs. These books do one job: they say a sad with no reason is still a real sad, and you're allowed to have it. Pick by your child's age and by what their low actually looks like. When Sadness Is at Your Door is a good place to start if you're not sure.
Any single book here delivers the whole promise on its own, so you might read exactly one and be done. The matching is what matters, and there are two ways to do it. First, age: a three- or four-year-old wants the books that turn a feeling into something you can see and point at, and a seven- or eight-year-old can sit with a book that doesn't resolve. Second, and more useful, what your child's low looks like from the outside. If they've gone silent and shruggy, the books that give sadness a face let them gesture at it without having to explain it. If their mood is dragging the whole house down with it, Virginia Wolf is about exactly that, including how badly cheering-up works. If they perform fine at school and collapse at home, Michael Rosen's Sad Book opens on a man explaining that his smile is fake, and that page does a lot of work. When Sadness Is at Your Door carries the start-here flag because it's the easiest to hand to almost any child on this shelf and it asks nothing of them — no reason required. That's a good place to start, not a first rung.