What about agents – is it worth having one?

Really it’s up to you. Literary Agents (they take on illustrators too) take about 15-20% for a book contract. Illustrator’s agents take about 25-40%. For this they usually have you on their own website, they field calls for you, negotiate fees, present your portfolio/website to clients, invoice clients, advertise (though I think you pay a % of this too) and promote your work, give advice and feedback and basically look after the business side for you. It’s not imperative to have an agent, but if you’re not business minded or would rather not deal with that  side at all then it would be well worth your while investigating. A great agency is www.illustrationweb.com 
The Association of Illustrators (in the US Graphic Artists Guild +1 212 791 3400) will have a list of agents and their contact details. Again, make sure your portfolio is up to date and see if you actually like who you are meeting as this is like any relationship – you have to get on, trust and respect each other.