Dear Agent -
When should you put published short stories in your query letter? There are paying markets that are relatively easy to get into, and non-paying markets with incredibly high standards. What do you (or agents in general) care about, and when does it seem like the author is trying too hard?
Dear Cory -
Agent Nathan Bransford addressed this very nicely in a post on his blog a few months ago, but it's worth discussing again.
Short Fiction/Essays/Narrative Non-Fiction: If you've published in a nationally known periodical (The New Yorker, F & SF, Ellery Queen Mystery Magazine, Realms of Fantasy, etc), by all means, list this in your query letter. If you have published in a well-known literary journal, online or print (Rain Taxi, Three Penny Review, Story, etc), list this in your query. If you have published in a paying online market (Fantasy, Strange Horizons, etc), list this in your query. There are also legit non-paying venues, but if you've published a short story in a non-paying venue, it really needs to be one that is well-known to agents and editors in the genre for which you are querying. (Your best friend's blog doesn't count.)If you have been a journalist or a columnist in a newspaper/online venue that more than a handful of people have heard of, list this in your query. (This does not apply if you wrote a column about, say, stamp collecting or gun cleaning. Use your head!) And if you have a blog, that also doesn't count as publishing credit. Even if 10,000 people read it. Sorry! But if you have a blog that 10,000 people read, it means that you already have an audience, so an agent probably does want to know about it. (Unless, of course, your blog is about, say, stamp collecting or gun cleaning. See above.)
Other Non-Fiction Credentials: If you are querying for a mystery or science fiction or fiction, etc, then listing your credentials as a technical or business or medical writer is pointless. Leave it off your query.
Poetry: This is a tough call. I'd say that if you have published a significant collection of poetry (like, in an actual book, yo!) through a legit publisher, go ahead and list it, especially if you are querying for literary fiction. But if you've had three poems published in your school paper, I don't want to know about it.
Screenplays: The only time that you should ever list a screenplay as a credit is if that screenplay has been purchased and turned into a film. Otherwise, it's the same as saying that you've written ten books that weren't actually published. It doesn't count as a publishing credit.
Self-Published Authors: This has been said countless times, but I'll say it again. Self-publishing doesn't count as a publishing credit. PublishAmerica, iUniverse, Lulu, BookSurge, etc...any mention of these names (or the nine-thousand various other self-publishing companies out there) will only work against you on your query. Leave them off.
Books: If you're a previously published author, through a legit publisher, you should list the title of the book and the publisher. Unless you've written an accounting book, and you're querying about fiction, for example. Again, use your head. You may have written 27 textbooks but they won't really have helped you hone your novel-writing skills, and the sell-through history on those books will have no bearing on any novel you may publish in the future.
Okay, I think that's about it. If I've forgotten something, let me know.
Hope that was helpful!