The book after *Throne of Fire* is The Assassin's Blade in the *Throne of Glass* series by Sarah J. Maas.
However, *The Assassin's Blade* is a prequel novella collection that tells the story of Celaena Sardothien's early years as an assassin. It's not a direct sequel to *Throne of Fire*.
The direct sequel to *Throne of Fire* is Heir of Fire.