The Paid Companion by Amanda Quick is $1.99
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I couldn't find the book description blurb but I think the Booklist review has a good summary:
Elenora Lodge is in quite a fix. Her stepfather lost her farm and all of her possessions in a mining venture, and her fiance dumps her faster than the proverbial hot potato. But Elenora is practical and pragmatic. So when Arthur Lancaster, earl of St. Merryn, offers her a position as a paid companion, she accepts. St. Merryn is in a bit of a fix himself. His favorite uncle has been murdered, and he's sworn vengeance on the killer, a mad alchemist intent on perfecting the ultimate weapon of mass destruction. Unfortunately, St. Merryn's fiancee has also dumped him, and his renewed status as one of London's most eligible bachelors is interfering with his quest for justice, hence his paying Elenora to pose as his new fiancee.