These are AMS's most popular titles right now. Plenty of good ones here. It's amazing how some older titles keep popping back onto the list!
The Mark of Athena by Rick Riordan
The Greek and Roman demigods will have to cooperate in order to defeat the giants released by the Earth Mother, Gaea. Then they will have to sail together to the ancient land--Greece itself--to find the Doors of Death.
Attack of the Vampire Weenies and Other Warped and Creepy Tales by David Lubar
Contains over thirty stories of vampires, ghosts, middle school teachers, and other scary things.
Breathe: a Ghost Story by Cliff McNish
When he and his mother move into an old farmhouse in the English countryside, asthmatic, twelve-year-old Jack discovers that he can communicate with the ghosts inhabiting the house and inadvertently establishes a relationship with a tormented, malevolent spirit that threatens to destroy both his mother and himself.
The Compound by S. A. Bodeen
Fifteeen-year-olf Eli, locked inside a radiation-proof compound built by his father to keep them safe following a nuclear attack, begins to question his future, as well as his father's grip on sanity as the family's situation steadily disintegrates over the course of six years.
Rapunzel's Revenge by Shannon Hale
Rapunzel, having grown up in a lovely castle with the woman she thought was her mother, is placed in a very tall hollow tree as punishment after her curiosity prompts her to climb the castle wall and look at the ruin of the world beyond her home, but she is able to escape and, with the help of Jack, embarks on a plan to free the land from the grip of the witch.
Brain Camp by Susan Chongmi Kim
Problem children Jenna and Lucas find themselves invited to Camp Fielding, where they are surrounded by other misfits whose parents hope will be changed into prodigies. When strange things begin happening around them, Jenna and Lucas team up to investigate, but what they discover is beyond anything they could have imagined.