The children meet Aunt Lucy,
This review is from: Lucinda's Secret (Spiderwick Chronicles) (Hardcover)
By now you have figured out that the Spiderwick Chronicles are actually one story split over several books much as the Tolkien story is not a trilogy but a one story split over three books.
In the last book we see that the children made up a lame excuse for their foray into the magical world around them.
Now they wonder why the Field Guide is of interest to every one and go to Aunt Lucinda for answers. As you recall Aunt Lucinda is in the Nut house, oops, rest home for eccentrics. They get more than they planned on.
We follow them as they discover the mystery of the missing Arthur Spiderwick. The adventure has a sort of Alice feel.
Be he alive or be he dead
They think they know
Which way Arthur fled.