The Party by Natasha Preston

Are you invited? In the heart of the English countryside, Bessie and her closest friends gather at a remote castle for a secret party destined to make this the best spring break ever. But when one of them dies, the party takes a lethal turn. Trapped inside as a deadly storm rages, it soon becomes clear Bessie and her friends are being stalked by a killer. As the body count rises, and secrets about the past unravel, Bessie begins to suspect that she doesn't know her friends quite as well as she thought. But as the party turns into a hunt, can Bessie make it out alive? This book is available in paperback.