Mount Doom is the 3rd chapter in the second book of The Return of the King (novel).
Plot
Frodo and Sam reach Mount Doom and the Crack of Doom. While Gollum tried to stop Frodo to destroy the Ring.
Synopsis
Yet just as Frodo is about to throw the Ring into Mount Doom, he succumbs to the Ring's power and refuses to let it go. Just then, Gollum, who had been following the pair still, attacks Frodo and bites off his finger with the Ring. Gollum gloats over getting his prize back, but loses his balance and falls into the lava below, taking the One Ring with him. The Ring is thus destroyed, freeing Middle-earth from Sauron's power. Then Mount Doom began to explode and the lava slayed The rest of the eight Ringwraiths. Frodo Baggins and Samwise Gamgee who were saved by lying on a rock.