FWIW (I don't have an agent but have been looking into getting one lately), once you have been working at this for a while you will have plenty of manuscripts waiting in the wings. While you are still agent-free, research your agents to find which of your manuscripts suit which agents. PB agents state what they like - eg character driven, quirky, lyrical, etc. Until you sign with an agent, you can have more than one manuscript out with different agents, but only send out your strongest ones. Be sure to keep good records so that if you get an offer, you can notify all other agents you have contacted. It's polite, and they may wish to counter-offer. It's fine to say in your initial query letter that you have other manuscripts too. Agents usually want to sign an author for the long haul (i.e. their career; not book by book), so it's good to say you have built up a body of work.
My two cents' worth.
Best of luck.