مَا انْتَفَعُوا بِالْكُتُبِ إِلَّا حِينَ أَحْسَنُوا قِرَاءَتَهَا.
They benefited from the books only when they read them properly.
“Ma ... illā” expresses restriction, yet the restricted element here is the time circumstance, not a person or a particular book. The sentence does not claim that no other students can benefit; it locates these students' benefit in a particular condition of reading.
Changing the wording to “Only the students read the books” would shift the restricted element to the agent. An identical restriction device can govern quite different semantic domains.