The Spiderwick Chronicles: by Holly Black and Tony DiTerlizzi

This is a modern-day meets a hidden fantasy world adventure. The three main characters discover a book filled with secrets about the magical creatures that co-habit our world writen by their great uncle Arthur Spiderwick. Unfortuneately no one in the magicle realm want humans to know about them, and thus both the good and the bad are determined to take the book from the children before they reveal of the knowledge to others. The evil troll Mulgarath, a master of shape-shifting and illusions, is especially determined to get the book and use the information therein to help him take over not only the fairy realm, but the world of humans as well. There is a good deal of suspense, as well as some action, as the children try to escape the goblins and outsmart Mulgarath as well as the cunning and lofty elves, while befriending brownies, pixies, and even a griffon.
Alongside the fantasy adventure, the story deals with the very real issues of a family going through a divorce creating a more serious undertone. However the fantasy plot takes precidence and the everyday issues are not fully developed.
Each of the books works more like a chapter in the story, rather than posessing individual plots, so, although it appears as a series, it is really just one fairly simple story and could easily have been published in one continuous volume. The reading level is probably suitable for mid-elementary.
The illustrations provided throughout are delightfull! I would also highly reccommend the illustrated field guide wich contains more full page color pictures and detailed decriptions of the different creatures we meet in the story.