Translation:We are able to read quietly in the library.
Because "tacite" is an adverb (=invariable, never declined),
and the adjective is "tacitus".
Probably "Biblioteca tacita" a quiet library, or a secret library (something that we don't talk about is one the meanings)
I don't know which meaning would be the right one, maybe it's context-dependent.
I wasn't sure we could use "tacitus" about innert things, but I've found "tacitae exceptiones" in the Gaffiot, so it seems ok.