some good novels to read at age 21?

Roads to Freedom trilogy
Middlemarch
Our Mutual Friend
Travels with my AUnt

By twenty-one, you OUGHT to have read a decent selection of the classics. If you haven't, I suggest ten to start you off. In chronological order -

The Iliad - Homer
Inferno - Dante
Paradise Lost - Milton
Don Quixote - Cervantes
Tom Jones - Fielding
Pride and Prejudice - Austen
Moby Dick - Melville
Bleak House - Dickens
Crime and Punishment - Dostoevsky
Catch-22 - Heller

OK, so technically the first three aren't novels, but let's not split hairs.

The Winter People by Jennifer McMahon
Bird Box by Josh Malerman
The Brief Wondrous Life of Oscar Wao by Junot Diaz
The Girl WIth the Gifts by M. R. Carey
The Basic Eight by Daniel Handler
Eleanor Rigby by Douglas Coupland
Inside the O Briens by Lisa Genova
What Was Mine by Helen Klein Ross
Suspect by Robert Crais
Wool series by Hugh Howey
Code Name Verity by Elizabeth Wein. (Technically YA but a lot of adults like it, too.)
And Then There Were None by Agatha Christie
The Haunting of Hill House by Shirley Jackson

The Long., Dark Teatime of the Soul, by Douglas Adams, is Worth a read by anyone, who likes a taste of the bizarre.

https://en.wikipedia.org/wiki/The_Long_Dark_Tea-Time_of_the_Soul

Dark Tower

You could re-read the novels you had to read in English classes. It amazing how some become "Wow!" when you don't feel the pressure of an assignment. You can also find out what the college and university English Lit. students must study. If you don't have to study them, you will be well grounded without the pressure. Besides, the English majors will have some interesting books for casual reading that they've read. Read Margaret Atwood's Alias Grace or The Handmaid's Tale.

If you need light fiction as a break from work or study, I suggest Agatha Christie's mysteries and P.G. Wodehouse's silly Englishmen or Dashell Hammet's Guys and Dolls stories, or Erma Bombeck's essays.

I would suggest non-fiction. History. Political science. Biology. Philosophy. Whatever grabs your interest in the library. History fascinates me.

Lolita.
Bleak House.

Blades by J. William Turner