Short answers, pulled from the story.
Fëanor son of Finwë King of the Noldor created three jewels that glowed with the captured light of the Two Trees of Valinor. These Silmarils were stolen by Melkor after he killed Finwë and fled to Middle-earth.
The first edition appeared in hardback by Allen & Unwin in 1977. Christopher Tolkien worked for several years after his father's death to construct a single text from numerous drafts ranging back to 1917 Book of Lost Tales.
Ilúvatar destroyed his forces and sent a great wave to submerge Númenor when Ar-Pharazôn landed on Aman. All died except those who remained loyal to the Valar while the world was remade and Aman removed beyond the Uttermost West so Men could not sail there.
A major influence on the text was the Finnish epic Kalevala especially the tale of Kullervo while Greek mythology appears in how the island of Númenor recalls Atlantis. Norse mythology influences characters like Thor who can be seen in Oromë fighting monsters of Melkor and Celtic influences appear in exile of Noldorin Elves borrowing elements from Irish legends of Tuatha Dé Danann.
Reviews at release were generally negative though the book topped The New York Times Fiction Best Seller list in October 1977. Robert M. Adams of The New York Review of Books called it empty and pompous bore not literary event of any magnitude while Time magazine lamented there was no single unifying quest or band of brothers for reader identification.