This review will contain spoilers!
In many sentences:
Fablehaven was an interesting book to read having just completed The Children of Hurin. While I enjoyed both, the contrast between the epic scope and the sense of doom prevalent in The Children of Hurin versus the much lighter tone and smaller scope of Fablehaven was a little jarring. Still, these differences made Fablehaven a very enjoyable book if you are looking for fast-paced fun set in our own world (just with real magical creatures).
In terms of the major themes, Fablehaven touches on courage and justice most prevalently. This is clearest when we consider the climax of the book. Kendra, having run from the Forgotten Chapel, leaving her family behind, decides to risk her own life by traveling across the naiad filled pond to speak to the Fairy Queen. Obviously, this is a courageous act and since it is the cornerstone of the book's outcome, I think it's reasonable to assume that the author wanted to show that courage can be rewarded.
Justice, however, is slightly less obvious but not less significant element. Rather than just looking at one example as we did above, here I think we can see its importance more clearly by following a chain of events. First, Seth breaks the rules multiple times by going into the woods. There, he meets and harasses the witch Muriel. This disobedience not only nearly gets them locked in the house for the rest of the trip, but also encourages Muriel's ill-treatment of Seth later on. Next, Seth captures a fairy, and turns her into an imp (though accidentally). The immediate punishment is, of course, being turned into a deformed walrus monster. Later, however, he is imprisoned by the imp he turned inside a glass bottle.
Kendra - who did not misbehave, use magic, or harm any magical creatures - is not only safe from the fairy retribution against Seth, but she is also safe from harm by Muriel's imps and other creatures. Ultimately, it is only because Kendra did listen and obey that she is free to go to the Fairy Queen and to return with the Fairy army. In other words, each of these actions had a fitting outcome, and all of these events were of primary importance to the outcome of the book. Justice, or a world in which there are just and fair consequences to your actions, is therefore key to understanding the world of Fablehaven. It will interesting to see if this same sense of justice is continued throughout the series as a whole
I'd like to move on from the above, however, because Fablehaven is too fun of a book to get bogged down in detailed discussions on justice and retribution. One of the aspects I really enjoyed was how realistically the characters behaved (at least, for the most part), considering their situation. For example, upon meeting Muriel and seeing her chewing her rope and being asked if he would like to stick his arm in a mysterious wooden box, Seth and Muriel have the following conversation:
"Are you a Witch?" "A man with a brave tongue should support his words with courageous actions." "This seems like something a witch would do" (39).
There aren't many books that would have a character take this approach when meeting a weird old woman, but I found it to be a refreshing and honest representation of a modern boy. This is also a pretty good example of how funny the book can be. While not quite as sarcastic as the Percy Jackson books, there were many moments I laughed out loud.
Another thing that surprised me about Fablehaven (considering the overall tone) was how scary the Midsummer Eve scenario was. Part of this, of course, may be the fact that I often was frightened of monsters in the closet as a kid, but at the same time credit must also be given to Mull for the vivid scene that he painted for us. Also, I love the fact that the safest place for the children was in their beds under the covers. Again, maybe it's just me, but when I was young I always thought "as long as I stay under the covers nothing can get me!" and Mull's depiction here nails that feeling perfectly.
To conclude, I just want to say that the scene of the battle, with the human size fairies battling Muriel, Bahumat and kissing the imps to restore the fallen fairies was, quite simply, awesome. The image of these beautiful creatures kissing hideous monsters to rescue them was so vivid in my mind that I was moved in a way that surprised me for a book like Fablehaven. Clearly this is a book I recommend and I look forward to reading the rest of the series.