to read (a book); to open (a book) to read
to unravel (e.g. a mystery); to discover (the truth)
歴史家は古文書を紐解いて、真実を究明した。
The historian unraveled ancient documents to ascertain the truth.
刑事はついに謎を紐解いた。
The detective finally unraveled the mystery.
彼は家族のルーツを辿るため、古い日記を紐解く。
He unravels an old diary to trace his family roots.