Q: If you're a first-time writer, what should you do when an agent requests a publishing history and author bio?
A: If you have no publishing history or credits, then just say so. Remember that if you're submitting a novel, the thing that matters most is the quality of the writing. Is it good? If it is good, then it doesn't really matter whether you've published 15 short stories or none. So why do agents ask for it? If an agent sees a bio with credits (and awards), they know they're dealing with a professional. The writing must stand on its own, but a a good bio may help your work get considered faster.
With nonfiction, you must have a platform to get a book published - meaning: Are you an expert in the field who can reach potential book buyers? If you want to write a book on horticulture, for instance, but have no magazine or journal publishing credits in this subject, you may want to get some before trying to sell a book-length project.
Q: In Guide to Literary Agents, some agencies state their preference on receiving simultaneous queries and some do not. What about those who do not specify? Is it acceptable to send them simultaneous queries if they don't specifically ask you not to?
A: Yes. It is "normal," for lack of a better word, for writers to query multiple agents at once. Agents who want an exclusive read will say so. If they do not, assume they accept simultaneous submissions.
Q: I understand that if you're contacted by an agency, you're expected to give them a three-week exclusive. If you send out multiple queries and receive multiple answers, what is the proper thing to do as far as the agencies you did not pick are concerned? Should you inform them that another agent has taken an interest?
A: First of all, if you are contacted by an agency, there is no guarantee they will want an exclusive. That is a possibility, though.
There's no easy answer here. Just be honest. If an agent contacts you and asks for a four-week exclusive read, you'll probably say yes. If a second agent calls and asks for the same, just tell them the truth. Mention that another agent has an exclusive read on it, and ask if they would like an exclusive after that if a deal has not been made.
Q: What is a partial?
A: A partial is a portion of your entire story. When an agent requests "the first 3 chapters" or "the first 60 pages," that is a partial. Agents will usually review queries, partials, and (finally) full manuscripts.