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Don't read while walking.
Literal
Walk-while book [obj-を] read [must not-てはいけない].
~ながら chains the simultaneous actions ('walking while reading'), and the whole clause is then prohibited with ~てはいけない ('must not / it won't do'). A typical safety-warning structure — exactly the kind of admonition you'd see on a station sign or hear from a parent. The polite form ~てはいけません is also common in posted signage.