The Disowned Queen's Consulting Detective Agency

When it comes to light that Octavia de Reine isn't a legitimate member of the noble House of Reine, she swiftly finds herself disowned. At first she takes this in stride...until her power hungry father and ex-suitor start vying for the house full of mysterious heirlooms that her doting grandmother left her. Little does her family know that she's destined to inherit an even greater legacy. Olivia hopes to follow in Granny's footsteps and become a famous detective, after all!

She'll face off against young noblemen, phantom thieves, and magical items corrupted by demonic forces. Oh, and one vengeful angel queen. Moreover, who's this suspiciously young marquis that seems so smitten with her...?

(Source: J-Novel Club)