In publishing, a collection is a book of short pieces (stories, essays, or poetry) all written by the same author or co-authors. Contrast this with an anthology, which is a book of short stories (or, less often, essays or poetry) written by different authors. A collection is usually a substantial book that will be the same length as a novel. Many literary award requirements stipulate that a collection has to be at least 40,000 words in length; however, publishers that handle collections usually want to see manuscripts that are at least 60,000 words in length. Some "best of" collections from prolific authors will be considerably longer. A small assemblage of stories or poetry that does not meet the length requirements for a collection is typically referred to as a chapbook.

Many anthologies are centered on a specific theme or topic, such as stories about Vikings or poetry written in Manhattan; giving the stories in anthologies some kind of thematic connection is important so that the book makes some kind of sense as a whole. Because collections are the work of a single author, there's a built-in sense to the book; in a fundamental way, the author is the theme. That said, a prolific author may very well divide his or her short works into collections according to genre lines: he or she may put all their cozy mystery stories into one book and all their military science fiction into another. That type of division makes good sense from a marketing standpoint, since those genres don't have much in the way of a shared readership. 

When putting a collection together, it’s very important to consider the flow of the short stories and poetry in the book: do these stories all make sense together? Do any of the stories seem redundant in terms of character, plot or theme? If so, you may need to either put some space between the stories or pick the one you like best and leave out the other.

The first three and last three stories in any collection or anthology are especially important. The first couple sets the tone for readers and gives them the sense of whether or not they want to keep reading your book. Even if the stories in your collection all concern different themes and explore different genres, if a reader doesn't like your first story, he or she is very likely to move on to something else. 

The final stories in a collection are important because they finish the experience your reader is having. Have you ever had a good meal that ended in a lousy dessert? Even though the main course and appetizer were excellent, the piece of burned pie or canned pudding left a bad taste in your mouth. The same goes for collections: you don’t want to end on an unsatisfying note. Start strong, and make sure you finish strong.

A big challenge with collections is that they're fairly hard to sell to publishers. Large publishers such as Random House are generally only interested in collections from bestselling popular authors or literary authors whose work has won major literary awards such as the Pulitzer. The reason for this is pretty simple: short fiction and poetry don't sell nearly as well as novels do, so if a big publisher is going to take a marketing risk on a collection of short stories, they want to have some confidence that it will sell because the author has amassed a significant readership.

There's serious irony here for genre authors: if you've been spending your time writing stories and focusing on honing your craft in the short form, you're probably a better short story writer than most bestselling novelists, but you're not a bestseller because you haven't been writing novels. And so the big publishers won't even look at your story collections. Fortunately, if you've been spending time publishing short stories in decent publications, small- and mid-sized publishers will be receptive to your collections.

But if you're a newer writer, getting a collection published with a decent small press is very difficult without having a track record of having sold to good magazines and anthologies. And getting that track record is hard: any given publication might only accept 3% of the submissions it receives. If nobody knows who you are as a writer, you either have to be very good, very lucky, or very patient in order to get published.

That level of patience and persistence in the face of rejections is difficult, and I've seen a whole lot of newer story writers and poets default to self-publishing: they decide to put it out on Kindle and call it a day. And while that's a perfectly valid choice to make, some who do this may be selling themselves short.

My own personal tactic is to sell as many individual stories or poems as I can before I put them together in a collection, though I do always include one or two new pieces to entice regular readers. I have several reasons for this: 

  1. I often get paid more for each individual story than I will get as an advance on my collection (because short story collections don't sell as well as novels, advances reflect that). I normally get at least $300 for a new story I place with a pro market, and sometimes a lot more than that, but I'm only likely to get a $300-$1500 advance on a collection that contains a dozen stories. If I sell all the stories first, though, I've made $4000-$5500 or more on all the writing. 
  2. Publishers tend to feel more confident about a collection of stories that have already sold to professional markets. 
    • Magazine/anthology publishers' exclusivity clauses get annoying here; for instance, I really wanted to include a particular story in my last collection, but according to the contract I can't re-use it before October 2017.
  3. Going through the editorial process at a magazine or anthology often makes the story better in some way, and so my subsequent collection is a stronger book than if I'd gone straight to putting the unpublished short stories together.
  4. Having my stories regularly published in good magazines and anthologies makes editors aware of me, and that makes it more likely that a publisher will be interested in buying my books.

A final note: if you've put a collection together and are planning to send it out to book editors, make sure that in your cover letter you call your manuscript a collection and not an anthology. Not using the proper terminology might seem like a minor thing, but it signals to the editor that you may be writing from a place of ignorance and that's not a great first impression.