This book happens to be one of those very, very rare cases in which the movie is actually better.
The basic story is that Miss Pettigrew--a middle-aged spinster--receives a new post as a governess in 1930s London. When she arrives, however, she finds no children. Instead, she meets beautiful young nightclub singer Delysia LaFosse. Delysia is trying to juggle three men and a burgeoning social calendar, and things are beginning to get too difficult. Decisions have to be made, and Delysia isn't sure she's up for it. Luckily, Miss Pettigrew finds that she can be of help.
The book is quite clearly a fantasy, something of a Cinderella story. Miss Pettigrew makes new friends, gets a makeover, and finds a happy ending. Nothing too difficult happens, and nearly everyone goes away better off. It's an adorable story, but frankly a bit light.
The movie, on the other hand, is stellar. Frances McDormand stars as Miss Pettigrew, and Amy Adams plays Delysia. They're supported by an excellent cast of gentleman (particularly Lee Pace as the brooding piano player Michael and Ciaran Hinds as lingerie designer Joe Blomfield) and Shirley Henderson as the poisonous Edythe DuBarry. The movie has more conflict--Miss Pettigrew has more to lose, and is forced to make tougher choices. Miss Pettigrew in the book tends to drift along in everyone elses's wake, whereas McDormand shows the steel that lies beneath Miss P's frumpy exterior. Adams's Delysia is a bit more scatterbrained than the one in the book--she's not as strong a female character in the film, and requires more rescuing--but she is still charming and lovable. Both Adams and McDormand are able to communicate volumes with a small look or tiny raise of the eyebrow.The fact that the film is set against the run-up to WWI also helps to put the characters in a more precarious position. The plot contains more conflict, but it all resolves rather well. In addition, the costumes, sets, and music are all gorgeous. They set the mood very strongly, as well as making the movie a pleasure to see. As usual, Focus Features has done an excellent job bringing a period piece to a modern audience.
On the whole, although I enjoyed the book, I'd recommend skipping it and going straight to the film.