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.I have booked my next adventure and started researching, and could pitch the idea of it, but obviously I cannot start on the manuscript until I've done the travel. Should I wait until I have written the manuscript before submitting to an agent, or can I pitch my idea based on the fact that I have a book in the same genre?
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.