Symbolism, Imagery, Allegory
Mirror, mirror, in the book, where's your symbolism if we look?
If you read closely, you'll see mirrors in a few key situations in Fablehaven. First there's the mirror that Kendra brings down to the pool, and which draws a suspicious amount of attention from supposedly ordinary insects:
After less than a minute, a hummingbird glided over to the mirror and hovered above it. Soon it was joined by a few butterflies. A bumblebee alighted on the face. Before long another swarm of small winged creatures crowded the mirror. (2.69)
We're no entomologists, but we're gonna go out on a limb and say that's atypical behavior for all these small winged critters. Later on it makes sense, since we learn that they're actually fairies (the kids just couldn't see them for what they were at the time) and fairies are very vain.
Next up, we see a mirror when Seth manages to catch a fairy and imprison her in a jar. He leaves the mirror in with her, thinking that it'll be nice since fairies like to look at themselves. Wrong. When the fairy transforms into an imp, breaks the mirror, and then escapes, Seth is puzzled. It takes Grandpa's explanation to clear up what this means:
"I am told Seth even left a mirror with the fairy, so she could behold herself after she fell. The fairies considered that act particularly cruel." (8.100)
Normally we wouldn't think about leaving a mirror somewhere easily accessible as a mean thing to do, but it has a special significance for fairies and imps. And as much as fairies love admiring themselves, they loathe witnessing their ugly transformations into imps.
So what do these mirrors symbolize? In both cases, the kids are ignorant about some aspect of fairy existence: in the first case that they exist at all, and in the second case that fairies can become imps if left imprisoned indoors overnight. Mirrors are thus associated with things-not-yet-known, of which there are plenty for Kendra and Seth in this book. It's an interesting twist, since mirrors often are symbols for truth—but in the mixed up world of Fablehaven, they instead indicate not knowing what's up.