19th century books seemed to have double titles frequently, like
Frank’s Campaign; or, What Boys Can Do on the Farm for the Camp Timothy Crump’s Ward; or, The New Years Loan, And What Became of It Moby-Dick; or, The Whale
Tolkien may have been riffing on that theme with The Hobbit, or There and Back Again.
Is there any reason for this style (today authors use subtitles fairly frequently)? When/why did it go out of style?
Depending on the genre, the “or” has been replaced by the “colon-subtitle” combo. If I go to Amazon and search for “self-help books,” my first four hits are as follows:
Get Out of Your Own Way: Overcoming Self-Defeating Behavior
Self-Love Workbook for Women: Release Self-Doubt, Build Self-Compassion, and Embrace Who You Are
Good Vibes, Good Life: How Self-Love Is the Key to Unlocking Your Greatness
You Are a Badass: How to Stop Doubting Your Greatness and Start Living an Awesome Life
Just from observation, the practice pretty much ended at the end of the 19th century and lasted only on lesser books.
A quick check tells me the only Dickens’ novel to have an “or” was Oliver Twist, or the Parish Boy’s Progress, from 1841. None of Trollope’s books did or George Eliot’s, and they both wrote in 1850s. On the American side, Nathanial Hawthorne didn’t use “or” and he started in the 1850s (except for 1828’s Fanshawe and that also lacked an “or”).
However, Harriet Beech Stowe used the “or” off and on from Uncle Tom’s Cabin, or Life Among the Lowly (1852) through We and our Neighbors; or, The Records of an Unfashionable Street (1875). And only two of Louisa May Alcott’s books from 1868 to 1880 had “or”.
But dime novels and weeklies continued to use an “or” straight through their end. I can find examples from the 1920s.
One need only go back to the '60s to find Principia Discordia: Or, How I Found Goddess And What I Did To Her When I Found Her, the Magnum Opiate of Malaclypse the Younger, Wherein Is Contained Absolutely Everything Worth Knowing About Absolutely Anything.
True, but I didn’t read the OP as wanting to limit the discussion to novels. It just says “books.”
That said, I can think of Slaughterhouse-Five, or, The Children’s Crusade: A Duty-Dance with Death, published in 1969. That one gives us an “or” and a colon.
Gulliver’s Travels , or Travels into Several Remote Nations of the World. In Four Parts. By Lemuel Gulliver, First a Surgeon, and then a Captain of Several Ships
When it comes to the “titles that summarize the entire plot of the book” genre, I don’t think it’s possible to top Defoe’s The Fortunes and Misfortunes of Moll Flanders, Who Was Born In Newgate, and During a Life of Continu’d Variety For Threescore Years, Besides Her Childhood, Was Twelve Year a Whore, Five Times a Wife [Whereof Once To Her Own Brother], Twelve Year a Thief, Eight Year a Transported Felon In Virginia, At Last Grew Rich, Liv’d Honest, and Died a Penitent, Written from her own Memorandums.