Possession by A S Bryatt
The story begins in the Reading Room of the London Library where the main character, Roland Mitchell, as a postdoctoral research assistant at London University, is conducting research on a famous Victorian poet, Randolph Henry Ash. Inside the thick, black book covered with dust, Roland finds two letters written by Randolph Henry Ash that are affectionate and imply an interest in another woman that is clearly not the poet's wife. Roland "steals" the letters and begins his obsession with solving the mystery of who the woman was and what was the nature of their relationship.
The book was thick and black and covered with dust. Its boards were bowed and creaking; it had been maltreated in its own time. Its spine was missing, or, rather, protruded from amongst the leaves like a bulky marker. It was bandaged about and about with dirty white tape, tied in a neat bow. The librarian handed it to Roland Mitchell, who was sitting waiting for it in the Reading Room of the London Library. It had been exhumed from Locked Safe no. 5, where it usually stood between Pranks of Priapus and The Grecian Way of Love. It was ten in the morning, one day in September 1986.