Graves, often citing Anacalypsis and other works by Godfrey Higgins (1772–1833) as his source, asserts in the book that many messiah-like "saviors" were crucified on a cross or tree before ascending into heaven.
"One thing is clear — the mythos of the Hindus, the mythos of the Jews and the mythos of the Greeks are all at bottom the same; and what are called their early histories are not histories of humankind, but are contrivances under the appearance of histories to perpetuate doctrines." (Higgins, Anacalypsis)
Here is Graves' main list, arranged chronologically:
Thulis of Egypt, 1700 B. C.
Chrishna of India, 1200 B.C.
Crite of Chaldea, 1200 B.C.
Atys of Phrygia, 1170 B.C.
Thammuz or Tammuz of Syria, 1160 B.C.
Hesus or Eros 834 B.C.
Bali of Orissa, 725 B.C.
Indra of Thibet (Tibet), 725 B.C.
Iao of Nepaul (Nepal), 622 B.C.
Buddha Sakia (Muni) of India, 600 B.C.
Mitra (Mithra) of Persia, 600 B.C.
Alcestos of Euripides, 600 B.C.
Quezalcoatl of Mexico, 587 B.C.
Wittoba of the Bilingonese, 552 B.C.
Prometheus or Æschylus of Caucasus, 547 B.C.
Quirinus of Rome, 506 B.C.
He also lists a number of other holy figures who took the form of men and then ascended into heaven, including:
Salivahana of Bermuda
Zulis or Zhule of Egypt
Osiris of Egypt
Oru of Egypt
Odin of the Scandinavians
Zoroaster of Persia
Baal of Phoenicia
Taut, "the only Begotten of God" of Phoenicia, inventor of letters
Bali of Afghanistan
Xamolxis (Zalmoxis) of Thrace
Zoar of the Bonzes
Adad of Assyria
Deva Tat of Siam (Thailand)
Sammonocadam (Sommona-Codom) of Siam (Thailand)
Alcides of Thebes
Mikado of the Sintoos
Beddru of Japan
Bremrillah of the Druids
Thor son of Odin of the Gauls/Norse
Cadmus of Greece
Hil/Feta of the Mandaites
Gentaut of Mexico
Universal Monarch of the Sibyls
Ischy of Formosa (Taiwan)
Divine Teacher of Plato
Holy One of Xaca
(Fohi) of China
Tien of China
Adonis son of the virgin Io of Greece
Ixion of Rome
Mohamud or Mahomet of Arabia.
The book claims that a number of these deities or god-men shared at least some traits of Jesus as described in the New Testament, drawing the strongest similarities with Krishna. For example, some figures had miraculous or virgin births, were sons of supreme gods, were born on December 25, had stars point to their birthplaces, were visited by shepherds and magi as infants, fled from death as children, exhibited traits of divinity in childhood, spent time in the desert, traveled as they taught, had disciples, performed miracles, were persecuted, were crucified, descended into hell after death, appeared as resurrections or apparitions, or ascended into heaven. Graves also devotes chapters to the pagan roots of baptism and the eucharist, and concludes that Jesus was not a real person.

Question: Who is the main source of information for this story?
Answer: Godfrey Higgins.
Alternative answer: Godfrey Higgins