which book do they appear in?
The Lord of the Rings
Orc
Orcs were brought into modern usage by the fantasy writings of J. R. R. Tolkien, especially "The Lord of the Rings". 
where did the idea of orcs come from Thomas Wright wrote, "Orcus was the name for Pluto, the god of the infernal regions, hence we can easily understand the explanation of "hel-deofol" are they fictional characters? Yes. It is a fictional humanoid monster
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Orc [SEP] Introduction
An orc is a fictional humanoid monster akin to a goblin. Orcs were brought into modern usage by the fantasy writings of J. R. R. Tolkien, especially "The Lord of the Rings". In Tolkien's works, orcs are a brutish, aggressive, ugly and malevolent race, contrasting with the benevolent Elves and serving an evil power, though they share a human sense of morality but technically are a race of elves themselves. Mythological monsters with names similar to "orc" can be found in the Old English poem "Beowulf", in Early Modern poetry, and in Northern European folk tales and fairy tales. Tolkien stated that he took the name from "Beowulf".