from Gutenberg:
Easy
Kidnapped by Robert Louis Stevenson
Call of the Wild by Jack London
Treasure Island by Robert Louis Stevenson

Medium
Lorna Doone by R. D. Blackmore
Heart of Darkness by Joseph Conrad
Two Years Before the Mast by Richard Henry Dana
The Three Musketeers by Alexandre Dumas père
Winesburg, Ohio; a group of tales of Ohio small town life by Sherwood Anderson
David Copperfield by Charles Dickens
Silas Marner by George Eliot
Babbitt by Sinclair Lewis
A Connecticut Yankee in King Arthur’s Court by Mark Twain
The Time Machine by H. G. Wells
The Picture of Dorian Gray by Oscar Wilde
Father Goriot by Honoré de Balzac
Tess of the d’Urbervilles by Thomas Hardy
The Red Badge of Courage by Stephen Crane
Ivanhoe by Sir Walter Scott
The Virginian, Horseman of the Plains by Owen Wister
Jane Eyre by Charlotte Brontë
Great Expectations by Charles Dickens
The Country of the Pointed Firs by Sarah Orne Jewett
Main Street by Sinclair Lewis
Pygmalion by George Bernard Shaw
The Playboy of the Western World by J. M. Synge
Little Women by Louisa May Alcott
My Antonia by Willa Sibert Cather
Madame Bovary by Gustave Flaubert
Ethan Frome by Edith Wharton

Difficult
The Pilgrim’s Progress by John Bunyan
Oliver Twist by Charles Dickens
A Portrait of the Artist as a Young Man by James Joyce
Crime and Punishment by Fyodor Dostoyevsky
The Portrait of a Lady — Volume 1 by Henry James
The Portrait of a Lady — Volume 2 by Henry James
Anna Karenina by Leo Tolstoy
Moby Dick, or, the whale by Herman Melville
Metamorphosis by Franz Kafka
A Tale of Two Cities by Charles Dickens
House of the Seven Gables by Nathaniel Hawthorne
The Scarlet Letter by Nathaniel Hawthorne
House of Mirth by Edith Wharton
Pride and Prejudice by Jane Austen
Wuthering Heights by Emily Brontë
The Return of the Native by Thomas Hardy