Don't read books in dark places — it's bad for your eyes.
Literal
Eye [to-に] bad [reason-から] dark place [at-で] book [obj-を] read [prohibition-てはいけない].
目に悪い ('bad for the eyes') gives the reason with から before the prohibition. ~てはいけない is the standard 'must not.' A classic parental warning — reason-first structure is natural in Japanese.