The Heart of What Was Lost is the fourth novel in Tad Williams' Osten Ard saga, following To Green Angel Tower and preceding The Witchwood Crown.

After Ineluki the Storm King's fall in "Memory, Sorrow, and Thorn", his followers, the Norns, flee the lands of men and retreat north to their ancient city of Nakkiga.

As the Norns make their way to their land in the Nornfells, the Rimmersman leader, Duke Isgrimnur, leads his army in pursuit, determined to destroy the Norns and their ancient Queen Utuk'ku once and for all.

The book is told from three points of view: Duke Isgrimnur of Rimmersgard; a Norn leader, Viyeki; and Porto, a Perdruinese mercenary.

Williams has tapped back into the dynamic that made "Memory, Sorrow, and Thorn" so absorbing.