Cloche and Dagger by Jenn McKinlay

Scarlett Parker flees Florida for London when she becomes famous on the internet for throwing cake at her married lover. She and her cousin, Viv, each own half of Mim's Whims, a hat shop in Notting Hill Gate. The business was left to them by their aunt, Mim. Upon arriving in London, Scarlett finds that Viv is missing, so Scarlett has to manage the shop herself, with help from Harrison Wentworth (who handles the finances for their endeavor) and Fee (an apprentice that Viv took on). When Scarlett finds customer Lady Ellis stabbed to death wearing only one of the shop's hats, the situation gets even more desperate, leading to a break-in at Mim's Whims and the realization by Scarlett that she has to find Lady Ellis' killer. Cloche and Dagger is a book filled with colorful characters, but the mystery aspect of the novel is not as well developed as I would've liked.