Hi, I'm working on my PhD in Folklore so I have a list drawn from my professors and colleagues. I'm assuming you're looking for indigenous or otherwise subaltern folklore and mythology? Folklore covers expressive culture in many forms, and everyone has some. This list focuses primarily on verbal art forms (my specialty) with some material culture included. It's also focusing on indigenous groups and ethnic minorities. I have recommendations for Mormonism or other better-known traditions, but I'll try to keep a narrower focus here. These are primarily North American, though there are a couple South American in here.

Alan Dundes: "The Morphology of North American Indian Folktales"

Jason Jackson: "Yuchi Folklore: Cultural Expression in a Southeastern Native American Community"

Americo Paredes: "Folklore on the Texas-Mexican Border" and "With His Pistol in His Hand: A Border Ballad and Its Hero"

John McDowell: "Sayings of the Elders: The Spiritual Life of the Sibundoy Indians" and "So Wise Were Our Elders: Mythic Narratives From the Kamsá"

Tom Mould: "Choctaw Prophecy"

Pravina Shukla and Henry Glassie: "Sacred Art: Catholic Saints and Candomblé Gods in Modern Brazil"

Pravina Shukla: "Costume: Performing Identity Through Dress"

Claude Lévi-Strauss : "Tristes Tropiques"

Richard Dorson: "Bloodstoppers and Bearwalkers; Folk Traditions of the Upper Peninsula"

John Roberts: "From Trickster to Badman: The Black Folk Hero in Slavery and Freedom"

Barry Jean Ancelet: "Cajun and Creole Folktales: The French Oral Tradition of South Louisiana"

Zora Neal Hurston: "Mules and Men"

Carl Lindahl: "Perspectives on the Jack Tales and Other North American Märchen."

Vance Randolph: "Pissing in the Snow and Other Ozark Folktales"

Dell Hymes: "In Vain I Tried to Tell You: Essays in Native American Ethnopoetics"

Don Yoder: "American Folklife"

There's way, way more than I can list here, but check out the IU Folklore resources page here: http://www.indiana.edu/~folklore/graduate/readings_formgenre.shtml

If you have something more specific you're looking for, let me know and I can point you in the right direction.

(Sorry for the weird formatting. I'm on mobile)

EDIT -- A brief clarification: Most of these are not compilations or comprehensive collections, mostly because for a book on folklore, particularly of such a massive geographical area, is not going to be of excellent quality. "Folklore" covers food, dress, narrative, dance, music, art, architecture, and more, so to find something comprehensive on all of that for two continents' worth of people groups isn't a reasonable goal. That said, many of these books can be found cheaply though resellers or through libraries, so you should still be able to access them on your budget.