Spinster and Spice by Rebecca Connolly

Spinster, sweet Spinster
Isabella Lambert is a nice person. She is biddable, polite, soft-spoken, and eager to please. She also happens to be a spinster, and one of the writers of the infamous Spinster Chronicles, where she is less polite, more outspoken, and may not please anyone. Izzy also has a secret that only one person knows, and that one person may hold the key to its success.
Sebastian Morton finds Izzy Lambert fascinating. She is the perfect mentor for his younger sister as well as a capable and charming friend for him. When opportunity strikes to spend more time with her, her secret becomes his. But will this secret, and others, ruin their connection?