Want a true confession? Until a few months ago I had never even heard of an author one-sheet. I had studied the craft of writing but didn't really know a lot 'specifically' about how authors presented themselves to agents and editors at conferences. I still have a great deal to learn, but after lurking the various writing related e-mail loops, and investigating samples etc. I feel I finally have at least a handle on what the one-sheet is intended for, when to use it, and when not to.
A One-Sheet is a Marketing/Memory Tool
When given only 15 minutes in front of an agent or editor as a conference appointment – with your nerves on edge, and the plot of an entire novel (or three) ready to trip off of your over-eager tongue in a ramble of nervous energy – the one-sheet is literally one sheet of information, either single or double-sided, filled with information about you as an author and whatever you are pitching. Having one with you is a way to prepare, to look professional, and keep yourself on track so that you don't run out of time before you get around to saying the things you 'intended' to say. Additionally, if the agent or editor you speak to is interested in hearing more from you, they may opt to keep your one-sheet to jog their memory about your pitch, and to glean your contact information from it. Note: one thing I have seen stated repeatedly is to not 'expect' them to keep it, (in other words, don't be offended if they don't) and to never push business cards, one-sheets, proposals etc. on anyone.
How Do You Know if You Need a One-Sheet?
1. Do you have at least one completed manuscript?
2. Are you ready to pitch it?
If you answered 'yes' to those two questions, then the answer is still only 'probably'. From what I have read, you can proceed to pitch to an editor or agent successfully without one. It is not an absolute requirement. If you are an author, but you don't have a completed manuscript and/or you aren't ready to pitch your work, you may opt to spend your appointment time asking questions. If this is your situation (like it is for me) then you don't really need a one-sheet yet.
(I still plan to take a single sheet of typed-up questions I have though, simply because I can't remember my own name when I get too nervous!)