Knife Edge by Andrew Lane

Teen Sherlock battles a monstrous adversary on a mission to Ireland with his brilliant brother, Mycroft. Young Sherlock is thrown into a tangled web involving a spiritualist whose powers have attracted the attention of governments around the world. At the castle where the medium is demonstrating his “gift,” Sherlock finds a household in turmoil. Servants and some of the guests are frightened but who–or what?–has terrified them so much that nobody will speak out? Young Sherlock must bring all his powers of deduction to bear in unraveling his greatest mystery yet.

Sherlock Holmes: Think you know him? Think again.