Night Watch is one of Mira's favorite Discworld books, and makes her cry like the sap she is. Partly a hardboiled police procedural, partly humorous fantasy, and partly political commentary, it is responsible for her crush on Discworld copper Sam Vimes.
The Les Miz Factor Edit
In addition, Mira is unable to listen to Les Miz with a straight face due to this book. This is because Night Watch is also a parody of of the story, as noted in these parallels (taken from the Annotated Pratchett):
Some of the parallels include the fact that in Les Miserables the plot concerns Jean Valjean, who is being pursued by an officer of the law many years before the start of the book/musical, which mirrors what happens to Carcer in Night Watch.
In LM, Jean Valjean is essentially a good man whose crime is the theft of a loaf of bread. Carcer is a murderous psychopath (who later claims that his original crime was stealing a loaf of bread).
Javert, the policeman in LM, is concerned only with justice, which he defines as the punishment of the guilty. Vimes, the policeman in NW, is equally obsessed by justice, but he defines it as the protection of the innocent.
In LM, Javert attempts to join the revolutionaries on the barricades as a means to betray and defeat them. Vimes organises the building of the barricades as a means of protecting the people.
Valjean tries to save a prostitute, Fantine, and when she dies he promises to take care of her daughter. Vimes is saved by a prostitute, Rosie Palm (who will later become famous for having "daughters").
In both LM and NW, a street urchin plays a role in the rebellion. LM's Gavroche dies, while Nobby survives.
Both rebellions (certainly in the musical version of LM) are "led" by impassioned revolutionaries in frilly shirts who take a long time to die.