Certainly there are more scriptures, more sagas, more reference books, more histories. These, at least, are the ones I come back to most often. If I could take about a hundred books to the desert island instead of just one, they would be these.

Prehistory to Indo-European & Early Europe & Asia:
Becoming Human: Innovation in Prehistoric Material & Spiritual Culture, ed. Colin Renfrew
Return to Chauvet Cave: Excavating the Birthplace of Art, the First Full Report, by Jean Clottes
Lascaux: Movement, Space & Time, by Norbert Aujoulat
Prehistoric Art: The Symbolic Journey of Humankind, by Randall White
Europe Between the Oceans: 9000BC-AD1000, by Barry Cunliffe
Ancient Europe, 8000 BC-AD 1000: Encyclopedia of the Barbarian World, ed. Peter Bogucki
Indo-European Poetry & Myth, by M. L. West
The Fall of the Roman Empire: A New History of Rome and the Barbarians, by Peter Heather 

Mesopotamia & Egypt:
The Epic of Gilgamesh
Before the Muses: An Anthology of Akkadian Literature
Ancient Egyptian Pyramid Texts: Allen / Faulkner
The Egyptian Amduat: The Book of the Hidden Chamber

The Complete Gods & Goddesses of Ancient Egypt, by Richard Wilkinson
Conceptions of God in Ancient Egypt: The One & the Many, by Erik Hornung
The Priests of Ancient Egypt, by Serge Sauneron
Ancient Iraq, by George Roux
The Oxford Illustrated History of Ancient Egypt, ed. Ian Shaw

Rig Veda
Hindu Myths: A Sourcebook Translated from the Sanskrit

Upanishads: Olvielle | Nikhilananda (1vol or 4vol)
Bhagavad-Gita: Schweig | Miller
The Gospel of Sri Ramakrishna

The Hindus, by Wendy Doniger

The JPS Jewish Study Bible
Traditions of the Bible: A Guide to the Bible as It Was Bible As It Was at the Start of the Common Era, by James Kugel, and its shorter edition, The Bible As It Was
The Five Books of Moses: A Translation With Commentary
Enoch 1, in Old Testament Pseudepigrapha
Book of Job: Scheindlin | Alter

How to Read the Bible: A Guide to Scriptures, Then & Now, by James Kugel
Yiddish Civilization, by Paul Kriwaczek
Holy Days: The World of a Hasidic Family, by Lis Harris

Tao Te Ching: Cleary | Red Pine | Roberts
Chuang-Tzu: Cleary | Palmer

In the Buddha’s Words: An Anthology of Discourses from the Pali Canon
The Shambhala Dictionary of Buddhism & Zen
Classics of Buddhism & Zen, translated by Thomas Cleary, 5vols

Greek & Roman
Complete Aeschylus
Complete Sophocles
Complete Euripides vols 1 2
The Homeric Hymns
The Iliad
Ancient Greek Religion: A Sourcebook, by Emily Kearns
Herodotus: The Histories
The Aeneid of Virgil: Mandelbaum | Ruden
The Georgics of Virgil
Tacitus: The Annals of Imperial Rome

Cassell Dictionary of Classical Mythology, by Jenny March
The East Face of Helicon: West Asiatic Elements in Greek Poetry & Myth, by M. L. West


The New Testament of the New Jerusalem Bible
The Gospel of Thomas, in Nag Hammadi Scriptures
The Desert Fathers: Sayings of the Early Christian Monks
The Conferences, by John Cassian
The Life of St. Columba, by Adomnan of Iona
The Little Flowers of St. Francis of Assisi
The Imitation of Christ, by Thomas à Kempis
New Seeds of Contemplation, by Thomas Merton
Dante’s Divine Comedy: Mandelbaum (1vol) | Singleton (6vols)

The Oxford Dictionary of Saints, by David Farmer

The Life of Muhammad, by Ibn Ishaq
Muhammad: His Life Based on the Earliest Sources, by Martin Lings
Essential Sufism, by James Fadiman

Poetic/Elder Edda: Orchard | Larrington | Hollander
Prose Edda: Byock | Faulkes
Volsung Saga

Norse Mythology: A Guide to Gods, Heroes, Rituals & Beliefs, by John Lindow

Siberia & Northern Europe
The Kalevala

The Mythology of All Races vol. 4: Finno-Ugric & Siberian, by Uno Holmberg
Shamanism: Archaic Techniques of Ecstasy, by Mircea Eliade

Celtic & Arthurian
The Tain
Early Irish Myths & Sagas
Perceval: The Story of the Grail, by Chretien de Troyes
Parzival, by Wolfram von Escenbach
The Quest of the Holy Grail

Oxford Dictionary of Celtic Mythology, by James MacKillop
The Holy Grail: Imagination & Belief, by Richard Barber

The Americas
Black Elk Speaks, by John Neihardt
Popol Vuh: The Mayan Book of the Dawn of Life and the Glories of Gods & Kings
Where the Two Came to Their Father: A Navaho War Ceremonial

Other Religious/Mythology Nonfiction
Patterns in Comparative Religion, by Mircea Eliade
The Myth of the Eternal Return: Or, Cosmos & History, by Mircea Eliade
God’s Almost Chosen People: A Religious History of the Civil War, by George Rable

Other History/Nonfiction/Literature
A History of the Byzantine State & Society, by Warren Treadgold
The Lost: A Search for Six of Six Million, by Daniel Mendelsohn
The Good Old Days: The Holocaust as Seen by its Perpetrators & Bystanders, ed. Ernst Klee
I Will Bear Witness: A Diary of the Nazi Years, by Victor Klemperer | vol 1 2
The Making of the Atomic Bomb, by Richard Rhodes
Working: People Talk About What They Do All They, and How They Feel About What They Do, by Studs Terkel

American Poetry
Walt Whitman
T. S. Eliot
Wallace Stevens
Robinson Jeffers