There is a type of loneliness in:
-The Hunchback of Notre Dame by Victor Hugo
-The Phantom of the Opera by Gaston Leroux
-The Scarlet Letter by Nathaniel Hawthorne
-A Christmas Carol by Charles Dickens (Scrooge’s loneliness, though, wasn’t known until AFTER he changed!)
-Oliver Twist by Charles Dickens (A good deal of the time, Oliver is quite alone – even among other people – though it has nothing to do with existentialism.)
-Anna Karenina by Leo Tolstoy
-Silas Marner by George Eliot
-The Secret Garden by Frances Hodgson Burnett (during part of the story)

There’s a rather profound loneliness (and separateness) in Watchers by Dean Koontz.

I’m sure there are many more books that could be named, but I hope this helps.

Robinson Crusoe

The perks of being a wall flower maybe?

Wuthering Heights: everybody’s sort of lonely in Wuthering Heights, but Heathcliff especially. The moor itself is lonely.

Rebecca’s nameless protagonist is pretty lonely, bewildered by the upper class she’s been thrust into and feeling like she can’t live up to the memory of her husband’s first wife.

Pollyanna has Aunt Polly, Dr. Chilton, Mrs. Snow, and Mr. Pendleton, adults who have been isolated for various reasons related to pride and misfortune, who are brought together by Pollyanna’s nosiness and innocence.

count of monte Cristo
i would say the outsiders but you said adult

A 1972 movie you might enjoy is “Tomorrow” It is about a bachelor Mississippi farmer. Robert Duval really captures the personality. Victor Hugo’s “Forgiveness and Redemption” is about a man on the run. He becomes wealthy but still has to keep a low profile because he is being looked for. Another wealthy loner is found in the novel “God Bless You, Mr. Rosewater”. Another movie you might enjoy is “Sling Blade” It is about a moderately retarded man who is gifted in making repairs of small engines. Also he has a no non-sense way of logical thinking. I guess its what some folks call “horse sense”

Cheers to Houanswer and to Advocate for two excellent choices.