HR Giger revealed himself through his paintings to be someone with an interest in JRR Tolkien's novels. Tolkien's most famous novels are The Hobbit, Lord of the Rings and The Silmarillion. The author turned out to also be a prolific illustrator and made numerous illustrations for The Hobbit, illustration the original cover. Tolkien also did some illustrations for Lord of the Rings
a) So far I can see that he named a painting series Mordor after Sauron's dark domain in Lord of the Rings, although the paintings appear not to noticeably relate to the novel.
b) There are the paintings named Lord of the Rings as well, but Giger simply liked the name for the paintings
c) Alien Monster IV features atrophied remnants of the scene of Gandalf fighting the Balrog as Khazad Dum is buried in
d) Giger's NY City XVI (1981) features elements from an illustration for the radio play version of Lord Of The Rings
e) Necronom II appears to feature ideas to do with the vulture headed Watchers from Tower of Cirith Ungol in Lord of the Rings.
f) "Demon" feature elements of the original cover of The Hobbit featuring mount Erebor.