I actually think The Mysterious Affair at Styles is a good place to start. It is more spare than her later novels, but you can see all the elements that characterize her mysteries. I didn't find it boring at all, and she does cleverly trick the reader, which is essentially her signature style.

"Language is like a cracked kettle on which we bang to make a bear dance, when we would move the stars to pity." Madame Bovary, by Gustave Flaubert.