There's a recurring question covered in blog posts by agents and authors that's often titled: "Is an Agent Worth It?"
I do not have an agent and have just received a contract for a 3-book deal. I would now say, without reservation, that an agent is DEFINITELY worth it.
I would give my left foot to have someone more experienced with more clout be in charge of this contract and the negotiations.
The story of why I am agentless is too long to include here, but I definitely want to strap on my armor and find an agent for any books subsequent to Book 3.
When is the best time for me to look for an agent under these circumstances? And, is there anything I need to know about querying under these circumstances -- anything that is different from the general agent-search advice?
For starters, if you (or any writer reading this) find yourself with an offer and no agent, email me and I'll put you in touch with people who review contracts for authors on an hourly rate. Most of them are former publishing company contracts department people.
As to your actual question: You'd do well to query now even though the agent will not be the agent of record for those books.
Never assume someone won't be interested in establishing a relationship with you early on.
Also, you can pay an agent from your contract proceeds directly. The publisher pays you, you pay the agent 15%.
If you are unable to secure interest at this stage, you'd query next when the publisher is talking about the option book for this contract.
This is one of those times when hitting a good writing conference will be useful since talking about your situation with an agent might be a lot easier than querying about it.