I have a novel coming out in May that has accidentally become a series. (The characters appeared in my dreams and made me write a second, and now a third. ) I obtained the publishing contract for book 1 without an agent, but if I want to find an agent for books 2 and 3, how much summary of book 1 do I need in my query?
None. You're actually querying for Book Two.
What you will need to tell the agent is whether this is the option book or not. In other words, does the publisher for book 1 have the option to acquire this second book before it can be shown to anyone else?
You should also undertake querying with realistic expectations. It's very hard to move a series to a new publisher; it's particularly hard if book #1 is with a small publisher and thus limited sales opportunities.