The thing about language is that words have a weird distribution. The most common 100 words show up in every single sentence, but then tons of "common" words show up statistically almost never. Like, "octopus" is a common word that is only going to be useful if you're talking to a marine biologist, or a three year old that's obsessed with octopuses, otherwise you're hardly ever going to use that word. There's a lot of words like that. "Spine" of a book? It's probably not "spine" in your target language.