I love you, shusu! Thank you so much for pointing me to it!
And therefore, I give my response:
Harry Potter and the Half-Blood Prince as written by Marion Zimmer Bradley
In this book, the Wizarding World and the Muggle World begin to make tentative efforts to merge in the face of a common enemy, the Death=Eaters.
Noting the growing psychic bond between himself and Harry, Voldemort decides to focus upon the uses of legilimency and occlumency as methods of destroying Harry. He forms among the Death-Eaters a cadre of skiled legilimentes who invade, not just Harry's dreams, but also his waking life, making it even more of a hell than it already is, in Potions class.
Dumbledore, against Snape's advice, gives Harry a powerful, magical artifact with which to defend himself from Voldemort's legilimentes. But, as Snape predicted, the artifact is too strong for Harry, being in fact imbued with the spirit of a long-dead legilimens whose mind now lives inside it and who wants Harry to be her 'host.'.
Hermione and Ron decide they must join forces to aid Harry in fighting off the mental influence of Voldemort and everyone else, so they form their own legilimency group. Though Ron turns out to be naturally gifted at mental wizardry, Hermione, being more willing to work at legilimency than either Harry or Ron, soon takes on a leadership role, which just ruins her and Ron's love-life.
At last, in the final battle that always happens at the end of the school year, Voldemort and his people launch an all-out mental attack on Harry, Ron, and Hermione. Our three young heroes prevail, but at the cost of Hermione's life, as she gives her last full measure and goes up in a pillar of flame, leaving Ron and Harry to mourn her and burning off Harry's wand hand in the process.
This results in Harry raging bitterly that he can do nothing, nothing...