Spiderwick Chronicles

You must explore the mansion and get the broomstick from the attic (use the ladder) Then go to the kitchen where there you'll find a wall with a crack in it next to the fridge. After you broke the wall you can use the lift to enter the study. Here you'll find a key on the desk. Take the key. Then you ca use the key to open the chest in the attic next to the telescope to get the field guide.
This achievement is story related.