Here are some Harry Potter blurbs I stole off Amazon. I think these are excellent guides for what a successful query should accomplish:
The Philosopher’s Stone (103 words)
Harry Potter has no idea how famous he is. That's because he's being raised by his miserable aunt and uncle who are terrified Harry will learn that he's really a wizard, just as his parents were. But everything changes when Harry is summoned to attend an infamous school for wizards, and he begins to discover some clues about his illustrious birthright. From the surprising way he is greeted by a lovable giant, to the unique curriculum and colorful faculty at his unusual school, Harry finds himself drawn deep inside a mystical world he never knew existed and closer to his own noble destiny.
Chamber of Secrets (157 words)
The Dursleys were so mean that hideous that summer that all Harry Potter wanted was to get back to the Hogwarts School for Witchcraft and Wizardry. But just as he's packing his bags, Harry receives a warning from a strange, impish creature named Dobby who says that if Harry Potter returns to Hogwarts, disaster will strike.
And strike it does. For in Harry's second year at Hogwarts, fresh torments and horrors arise, including an outrageously stuck-up new professor, Gilderoy Lockheart, a spirit named Moaning Myrtle who haunts the girls' bathroom, and the unwanted attentions of Ron Weasley's younger sister, Ginny.
But each of these seem minor annoyances when the real trouble begins, and someone--or something--starts turning Hogwarts students to stone. Could it be Draco Malfoy, a more poisonous rival than ever? Could it possibly be Hagrid, whose mysterious past is finally told? Or could it be the one everyone at Hogwarts most suspects...Harry Potter himself?
Prisoner of Azkaban (110 words)
For twelve long years, the dread fortress of Azkaban held an infamous prisoner named Sirius Black. Convicted of killing thirteen people with a single curse, he was said to be the heir apparent to the Dark Lord, Voldemort.
Now he has escaped, leaving only two clues as to where he might be headed: Harry Potter's defeat of You-Know-Who was Black's downfall as well. And the Azkaban guards heard Black muttering in his sleep, "He's at Hogwarts...he's at Hogwarts."
Harry Potter isn't safe, not even within the walls of his magical school, surrounded by his friends. Because on top of it all, there may well be a traitor in their midst.