Anthony Gilbert 
Death in Fancy Dress [EPUB ebook] 

When Tony Keith and his adventurous friend Jeremy Freyne arrive at Feltham Abbey, they find themselves entangled in a web of mystery and danger. As the Abbey prepares for a grand costume party, secrets from the past threaten to resurface and tear the family apart. With blackmail, forbidden romance, and a cast of colorful characters, this classic tale will keep you guessing until the very end.