I am constantly being told that in this world of author's self-promoting to get their work noticed, many authors are collaborating in a 'Unified Brand' anthology.
Writing a short story featuring your main characters is especially effective tool to introduce new readers to the other books.
A short story might sound easy, but having been invited to contribute to a Cosy Mystery Anthology, I am finding this hard work.
The task is to paint your characters, set the scene, devise a mystery, add clues, red herrings and a denouement - all in the space of a few thousand words! Keep it simple seems to be the best option, but omit backstory other than what is absolutely necessary, although include some so your characters don't become two dimensional, as this defeats the object, which is to engage readers and generate interest in the full length novel.
Another benefit is whenever the anthology is featured on social media and blogs, your name will also be listed as a participant, so the reach is far wider than going it alone. There is a downside though, for me anyway, in that I worry everyone else's story will be far better than mine and in this world of 'one chance to impress', I may get consigned to the reader's reject pile!
Ah well, nothing ventured nothing gained I suppose.