Liar’s Key is the first book I’ve read about Sharpe and Donovan even though it is the sixth book in the series. The good news is, you don’t really need to read the previous books to enjoy this one, but if you are really interested in the series, I would say start at the beginning. I think there is a lot of history that could make Liar’s Key even more enjoyable.
Agent Emma Sharpe works for the FBI on a special task force that deals with art crimes. Her family has also been known for doing investigations into art, since her grandfather had started an art detective agency decades before she was born. The FBI was able to convince her to come work for them.