If you're already happy with the press for your first book, then I don't think there's any reason to include the agent for the second book. Can the agent take you on for the other project? The one he/she read and liked?
Having an agent even when you got the publisher yourself can be helpful in the sense that the agent can help negotiate the contract and try to get you the best deal; however, if it's just contract review you don't need an agent for that. You can use a literary lawyer instead, and if you join the author's guild (for a small annual fee), they have lawyers on-staff who can review your contract, cost included in your membership (and no 15% commission!). Even with an agent on your side, it's not a bad idea to get an outside lawyer to review your publishing contract (even to review your agent contract).
If you have a new project the agent will take, then I would use the agent for that other project. Then you can get the real value of an agent, which is potentially getting you in with a larger press that does not accept unagented submissions.
I'm not an expert, though, so hopefully some other people will drop by and have more thoughts on the issue.
And congratulations on having two books published and a third with an agent! :-)