I don't know what you mean by "two books", but if you're talking about the storyline, there will be a new storyline created by us, but set in the same universe (with of course the due familiar faces).
Through the course of the storyline there will be all the stuff that made the books fun (Iris messaging, for instance) and we'll be adding some stuff too. So it's not exactly a translation of one of the books into m&b, it's more like a new book set in the game.
I think it'll be fun, and since we're planning on making the story pretty massive (you'll go through new york, camp halfblood, the underworld and all that), it won't be a one-day mod.
That, of course, is for the single-player campaign.