I have a travel book currently under contract with a publisher, due out at the end of 2009. It was a contract I negotiated myself, but I would now like an agent in order to grow my career.
You can pitch the idea, but don't get your hopes up for representation based on your pitch and your past book alone. It's not impossible - not many things are impossible - but your first book would need to have sold fairly well in order for an agent to take you on for the second without seeing a manuscript.
I say it's not impossible because I've taken on authors on the strength of their first book and their ideas for a second. But travel writing is a tricky genre (in Australia, at least - and I'm presuming you're Australian). I don't take it on because I've found it too hard to place in the past. Of course, if the writing were amazing, I would reconsider. But it would need to be amazing. You'd also need to have a great idea for the second book - something that's sufficiently different to other books on the market to make the agent (and, then, publisher) think it's worth taking on.