Fablehaven

Fablehaven by Brandon Mull is the first novel that I read that gave the idea that fairies weren't all fairy dust, cute wings, and springtime.
       Seth and Kendra Sorenson were caught unawares when they found out that their grandparent's large farm was actually a preserve for the endangered and hidden species of our world:  the magical and dangerous.  Now they're up to their necks in fairies who turn into imps, egocentric centaurs, Frito-lay-eating and television-watching satyrs, devious demons, shape-shifting dragons, ancient wizards, a quest for the five keys to the demon prison, and an evil society who knows no scruples.  But what is a fantasy story without romance, death, and fairy royalty?  Kendra and Seth will live to explore another day, if only they can figure out how to stop the rising Morning Star.

I believe that these books are Brandon Mull's best.  Sure, there are many different artifacts you have to keep track of, as well as the names of various enemies inside the Society of the Evening Star, but it all ties together nicely.  On top of that, the characters are likable and believable:  none of them are perfect or immune to human emotions.  In fact, more often than naught their actions lead them into more danger then they would've been in if they hadn't acted, if that makes any sense.  The characters are human, therefore, you can connect with the story better.  You find yourself agonizing whenever the characters (or, one character in particular) is going to do something that you know is rash or doesn't heed given directions. 
       Another thing I liked about these books are that you know no more than the characters do.  In some books you cut away to what the enemy is plotting, but in these books you can only see what Kendra (and sometimes Seth) are seeing.  You cannot see what their enemies are planning, so when the surprises come, they come with, well, surprise.  Also, the action comes fairly steadily, so rest assured -- there is at least one grand battle scene in every book (except the second, in which there is a scene involving sneaking somewhere).
        The books should be read in this order:  Fablehaven, Fablehaven: Rise of the Evening Star, Fablehaven: Grip of the Shadow Plague, Fablehaven: Secrets of the Dragon Sanctuary, and Fablehaven: Keys to the Demon Prison.