Eliot's note about the poem's title:

Eliot, in his first note to the poem praises Jesse L. Weston's From Ritual to Romance and Sir James George Frazer's The Golden Bough (although not naming him as the author.) Eliot wrote that the title of the poem was suggested by Weston's book. Weston often mentions a connection between a wounded Fisher King and his devastated domains which she refers to as the Waste Land (two capitalized words.)

This site has an example of Weston's use of 'Waste Land'. There is also an except from a letter where Eliot politely insisted that the poem's title was three words.

As noted elsewhere, The Waste Land was not the original title for the poem. Also, parts of The Waste Land were written in draft form prior to the 1920 publication of From Ritual to Romance and so care should be taken when considering how much Eliot used From Ritual to Romance as the plan for The Waste Land.

The Golden Bough:

Among the things that Sir James George Frazer wrote about was the myths scattered throughout the ancient world that gods, or men endowed with divine powers, died during the winter months and that the gods must be resurrected in the spring to allow the land to become fertile again. He also wrote about how the performance of ancient rituals was derived from mankind's belief that they could influence nature to cause this resurrection. Eliot pointed out some important sections of Frazer's work dealing with these themes.

From Ritual to Romance:

Jesse L[aidlay] Weston, who long studied and wrote about the Grail legends, was struck by the fact that there was no Christian story of Joseph of Arimanthea and the Holy Grail to account for the Grail romances. After reading Frazer's The Golden Bough she was struck by the similarities between the nature cults written about by Frazer and the Grail legends. One such was the connection of the wounded Fisher King with the devastation of the land. She concluded that the romance stories of the Grail were derived from ancient rituals and provided her evidence, sometimes scant, in From Ritual to Romance (1920).