First, this isn't quite the proper forum for this question, but I'm sure that the forum mod will move this over to a better location in a bit.
Well, before you can answer this question, first you have to decide a few things.
1. Is Book 1 of the trilogy complete within itself (all major plot threads introduced in THIS book are completed satisfactorily and most questions answered, so that the next book either deals with different characters or a different aspect of the same story?)
2. Is each of the other books complete within themselves so that they can stand alone as single title novels if required?
I ask these questions, because it depends on how you approach a publisher with the trilogy. If it will take all three books to wrap up the main plot line, a publisher might be leery of taking them on without any prior readership of your work. If the first book of the trilogy doesn't sell through (or just barely sells through) the advance, they might hesitate to publish the second and third, because a reader must have read the first book to buy the second.
If they can all stand alone, and you're just selling a continuing saga, then it's much easier, but you'll probably be selling them the first and second as a two-book deal, rather than the whole trilogy.
As for what to say in a query, I'd put something like:
While the first book, My Bestseller, does stand alone, and wraps up all introduced plot threads, it is intended to be part of a trilogy.
If it will require all three books to finish the plot, then you can put:
This first book in a trilogy, My Bestseller, wraps up most of the plot threads introduced, but several subplots are left open. They are picked up again in the second and third books to create a comfortable "saga" feel.
I will say that it will be better for your purposes if each book can stand alone. It's not required, but it's easier to sell.
As for a preface and illustrations, these are better discussed after they decide they want to publish the book. Do you mean a preface by an established author that might draw in other readers, or something written by you in the manner of a "Dear Readers" letter? If the illustrations are done by you (such as a map of the world or such), it's easy to handle afterward. Sometimes the publisher will like to include that, but don't make it a make or break issue, because illustrations are expensive to produce adequately in a book, and if you want to use someone else's illustrations, there are copyright issues that need to be discussed.
Sell the book first, deal with the details later, IMO.
Does that help?
