A couple weeks after I read this, I was still thinking about the original French sentence, and I realized that there's assonance between "livres" and "amis" and that this emphasizes this type of clause. "Livres" ("books") is the subject nominative, and "amis" ("friends") is the predicate nominative. Both words have the same semantic weight, so to speak (nouns on opposite sides of a copulative verb), and the assonance between them indicates this balanced relationship.
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Almost as a side-note, I'll add this: the entire sentence (describing Jean Valjean in the guise of Monsieur Madeleine) is "He loved books; books are remote but reliable friends," and this provides a direct contrast with Javert, described two chapters later (in Chapter V. "Dim Flashes of Lightning on the Horizon"): "In his rare spare time, although he hated books, he would read...."