>>Just wondering if it's worth getting one for a book through a small publisher?<<
I agree with Uncle Jim--you don't need an agent at this point (also, unless your publisher paid you a competitive advance, it's not very likely an established agent would be interested).
For your next book...In my opinion, if you want to sell fiction to a large publishing house, you need an agent. The number of imprints that will work with unagented authors is small and always dwindling, and those that are willing to look at unagented submissions give them very low priority and take ages to respond (I get sooo frustrated when I see posts from writers who've been waiting a year and a half, two years, or even longer to hear back from Tor or Baen, when they could have used that time to search for a good agent who'd be able to get an answer in a matter of weeks--assuming, of course, that the writer's work was marketable). Yes, novelists still do get bought "over the transom", but not terribly often. Again assuming that your work is marketable (and I say this because, quite honestly, most people's is not), your odds of success are infinitely improved by having a reputable agent.
If you want to stick with independent publishers, you don't need an agent, at least at the submission stage, because so many of them are still willing to work direct with authors. Many indies do pay competitive advances, so if you were to get a contract offer from one of these you'd then be in a good position to find an agent to negotiate the contract for you.
>>What does an agent do, besides sell books and negotiate contracts? Does he set up appearances, do press releases, stuff like that?<<
I think that the industry is slowly starting to move in that direction, and in the future we'll see more and more agents taking steps to give their clients the publicity push that their publishers may not provide (although I'm sure that if this happens it will work out heirarchically, as it does with publishers, with more lucrative or more exciting clients getting the push and others left to sink or swim as they may--thus not addressing but only worsening the plight of midlisters). But right now, most agents only agent, and an agent who claims to do PR should be regarded with suspicion--it may be a way to pry open your checkbook.
In addition to placing your book(s) with a publisher and negotiating contracts and advances, an agent sells subsidiary rights (foreign rights, dramatic rights, and so on--for some writers, this is very lucrative), serves as your advocate with the publisher, tracks and manages publisher payments (and pursues them if they don't pay), works with clients to develop projects, provides career guidance and advice, and may help you polish your work for submission. Once you've a commercially published book or two under your belt, your agent can sell work for you on the basis of a proposal and a sample, which means you aren't writing on spec anymore. This comes with its own challenges, but the security is nice.
One of the best things about having an agent (IMO, anyway) is that it makes it possible for a writer to do less thinking about the business side of writing. That's not to say you don't need to keep on top of things, but it does free up a lot of time and energy.
- Victoria