If you get your work published and intend to write sequels, should you tell your agent if asked just what you have in store? How obligated are you to spill the beans?
Why would you want to hide it? Legally, you would have to check your contract, but I doubt it would obligate you to tell her about any sequels. Morally, in a business relationship like that, I think you should be upfront.
I would suspect if your first book is good enough to have garnered an agent, they would probably be thrilled to know there are more in the offing. Unless, of course, you plan to take them elsewhere?
Sure, but I mean would the author need to share the details about the book, the plot, the ingredients?
I would telephone every day with every single fresh idea that sprang to mind, my imagination is vast, always ring my agent to spread my ideas in case I fell under a bus, off my bicycle or stopped breathing from natural causes. She could write it all down, all my ideas at this point, but she'd find my clause in the paperwork about not stealing my stuff in death, a win win thing.
A long conversation with a human being. I hate those. But that's it, really. I would be a little concerned about my ideas getting out in public, though, inadvertently spoiling the book. But that's a tiny matter because I'm sure he or she would be careful about disclosing details. Especially since it's in their best financial interest to keep it all under wraps.
The idea of a reputable agent stealing your story idea, or putting it in the public domain, is ludicrous. So only query reputable agents, and you don't have to worry about any of this.
If you want your agent to sell your first book, with the potential for selling future books to the same publisher, your agent needs to know the basics of what's in the later books so she can pass that along to interested editors. When my agent has sold series books for me, I've had the first book written, and then a brief synopsis of each of the later books. It's not about our contract or any obligation, it's just my own self-interest. I want her to find the best home for my work, so she needs to know what my work is. Remember that ideas are a dime a dozen--no one's likely to steal your idea, and even if they did, they'd write a totally different story than you will. Your agent works for you; give her the tools she needs to do her work effectively.
I agree with the people who say you should be upfront. I never had a working relationship with a literary agent, which I applaud anyone who has had a working relationship with one. I'd rather self-publish than traditionally publish, but I don't hate on those who traditionally publish their work. We each have our own paths. I can say from a business and even a personal standpoint it is best to be upfront for the most part, but that depends. I don't know the current relationship you have with your agent or if you completely trust your agent, which in that case you may want to look for another agent. Go in with caution if you feel unsure. Plan out how you might say it or give out certain details but not others. Make it sound interesting in the little detail you provide and the mystery you hint at. That's really the only suggestions I can give. I hope I've helped.
At the very beginning (in a query, say), I'd reckon just a rough idea of what the series will entail would be enough. So, for Harry Potter, you'd say that the subsequent books in the series "deal with the return to power of the Dark Lord, and the subsequent battle to defeat him once and for all," or something along those lines. Any specifics they need to know, you can discuss later.
I learned in film school, screenwriting class, that people are impatient nowadays so by giving your agent a logline like @terobi stated would help. In general, a logline is a short one sentence synopsis of your story, a rough summary of your basic plot. Drive home what, if anything, makes it unique.
The thing is, if you're querying, you are selling the first book and only the first book. The entire contents of the query should be about that first book, and make the agent want to read it. Sequels, series, etc. should be an afterthought, no more than a single line clarifying the rough direction the sequels might move in. I assume if you manage to get an agent based on that query, that they likely won't want a full rundown of every plot-point, but they'll likely want a bit more information than the query gives. How much you're actually obliged to tell them will presumably depend on your working relationship - but your agent and your editor are gonna be reading it eventually anyway, so why wouldn't you bounce your plans off them first if they ask?
A couple of writer friends of mine told me that an agent likes hearing that you are working on sequels and it can be a selling point for them. The agent invests time and money in selling you, and I think it's a good idea for them to know what you have in the offing. For me, it's easy: my book resolves with many questions. A sequel is naatural. BUT, if you don't have a sequel in mind, don't misinform them. Peace, Tex