After authors such as Ann Radcliffe and Matthew Lewis popularized Gothic fiction in the late eighteenth and early nineteenth centuries, hundreds of lesser known Gothic novels and condensed re-writes of better known Gothic novels were published in an attempt to cash in. This largely died down by the 1820s, but the large number of forgotten novels published by Minerva press (which also published Radcliffe's classic, The Mysteries of Udolpho) is a testament to the massive popularity of Gothic novels at the turn of the nineteenth century. Indeed, many of these "trade Gothic" works can be bought from Zittaw Press, Udolpho Press, and Valancourt Books. Thus, this trope is Older Than Radio.
The Da Vinci Code remained on best-seller lists for an obscene number of months, resulting in many copycat quest novels.
The incredible success of Harry Potter has led to a glut of children's fantasy and, while it isn't the first school for wizards, it is certainly the inspiration for many.
On a positive note, Harry Potter's success also persuaded authors and publishers to write longer and more complex young-adult literature.
When Stephen King published The Green Mile in serial format, lesser-known horror writer John Saul attempted the same thing with The Blackstone Chronicles. It didn't work as well.
The series caused a boom in the YA vampire genre. Notable examples include P.C. Cast's The House Of Night series, Richelle Mead's Vampire Academy series, and Melissa de la Cruz's Blue Bloods series, each having a wildly different take on the vampire mythos. Barnes & Noble now lists a new special category in the teen section: "Teen Paranormal Romance". It's pretty obvious where this comes from.
Just about every High Fantasy setting has its roots in J.R.R. Tolkien's Lord of the Rings. Sometimes they're knock-offs of works that are themselves knock-offs of Lord of the Rings.
Jasper Fforde pokes fun at this phenomenon in The Well of Lost Plots: A Thursday Next Novel. While Thursday is exploring the Well of Lost Plots, where books and characters are created from scratch, a Mr. Exposition explains to her that, when one character is written with a particularly forceful or distinctive personality, characters-to-be are affected by that and take on those traits. A side-effect of Daphne Du Maurier's Rebecca, for example, is that hundreds of impressionable characters imitated the creepy and possibly psychotic lesbian housekeeper of the story, which results in, for Jurisfiction, an army of Mrs. Danvers clones. At the end, he offers Thursday, "Can I interest you in a wiseoldmentor figure?"
While Tom Clancy was not the first guy to do the techno-thriller, he spawned a lot of imitators.
Somewhat to Sir Arthur Conan Doyle's chagrin, Sherlock Holmes arguably opened the floodgates for modern mystery and detective fiction, as detectives like Hercule Poirot, Nero Wolfe, and Inspector Morse all followed in his footsteps in one way or another. Holmes even provided a key inspiration for Batman's status as the DC Universe's greatest detective.
The UK and Ireland have recently seen a surge of popularity for "misery lit" books based on stories (some true, some not) of childhood abuse/Parental Abandonment etc. They all look exactly the same- a mostly white cover with a photo of a big-eyed child and heartstring-tugging title in twirly, bright lettering- occupy entire shelves in shops and seem to be competing with each other to see which can be the most depressing. Possibly launched in America by A child called it by Dave Peltzer, which then brought the craze to Britain and Ireland when it was released there.
Many bookshops now consider these a legitimate genre and have a section devoted to them, often called "Tragic Lives".
Philippa Gregory's Tudor-era historical romance novels (starting with The Other Boleyn Girl) jumpstarted a new wave of imitators set in or around the reign of Henry VIII (a trend exacerbated by the TV series The Tudors).
The Zombie Survival Guide and its companion World War Z have provided a lot of the momentum for the surge in zombie fiction. Works like Pride and Prejudice and Zombies have their origin in these.
After the success of Gossip Girl and the subsequent TV series, many more novels about rich white teenage girls (with a Token Minority or two) in private schools have been made. Some of the imitators include The Clique, the Private series, and Pretty Little Liars.
The Kimani Tru series, books about African-American urban teens, now has many imitators.
Almost everybody knows of Milton's Paradise Lost. What many people don't know is that Dutch writer Joost van den Vondel published De Lucifer, a play with the same basic plot, roughly four years before Milton even started writing his poem. While it's doubtful that Milton knew enough Dutch to fully understand the play, it's no stretch to say that he was inspired by the premise.
R.L. Stine's success with Goosebumps led to dozens of similarly named series being published including Bone Chillers, Deadtime Stories, Shivers, Spinetinglers, Spooksville, and Strange Matter.
At one point in the mid-nineties you couldn't turn around in a British bookshop without tripping over a "comic fantasy" with a Josh Kirby style cover. All they proved was there is only one Sir Terry Pratchett.
After the success of Don Pendleton's The Executioner books, a flood of copycat vigilante justice series jumped onto the bandwagon, with names like "The Destroyer" (which lasted the longest), "The Butcher", "The Penetrator", "The Liquidator", etc. Oh yeah, and a little comic book by Marvel called The Punisher.
