Learn Italian: Nessuno stava decifrando i documenti preziosi su quello scaffale. - Nobody was deciphering the precious documents on that shelf.

Because nessuno (nobody) comes before the verb. In Italian, if a negative word like nessuno, niente, or mai comes before the verb as the subject, you do not use non. If the negative word came after the verb, you would need the double negative: Non stava decifrando nessuno.

Why use stava decifrando instead of just decifrava?

Stava decifrando translates to "was deciphering." It uses the imperfect tense of stare plus the gerund (the -ando ending) to emphasize that the action was actively in progress at a specific moment in the past. You could use decifrava, but this form makes it much clearer that it was an ongoing process.

Why is it quello scaffale and not quel scaffale?

The adjective quello (that) changes its ending to match the definite article (the word for "the") that the noun would take. Because scaffale is a masculine word starting with an "s" plus a consonant, it uses the article lo (lo scaffale). Therefore, quello acts like lo and keeps its full "o" ending, becoming quello scaffale.