Lots of folks carry a book or an e-reader with them every day. Some tote around the Bible. Some a tawdry romance novel. The only book I regularly carry around (because I try to read it once a year) is On Liberty by John Stuart Mill. My edition was found in a used bookstore in Somerville Massachusetts. It was published in 1947 and has that "worn book" look to it. A yearly reading of On Liberty is something of a tradition among British citizens and it is easy to see why.
In America the word liberal has become something of a swear word. Conservatives foist it upon their ideological enemies and even those people don't use it, preferring the term "progressive." But before this name calling became the rage, "liberal" had a specific meaning: one who believes in the liberty and freedom of the individual as the building block of society. In its original meaning the term contrasts not with conservative, but a communitarian, who believes that people are part of groups and these groups or communities are the fundamental building block of society. The difference is important--for the liberal, governments exist at the whim of the individual, their authority flowing from the rights given up by each person for the benefit of living in an ordered society. For their opposite--the government doles out rights and freedoms to the individual. It is not a strictly liberal v. conservative split. In fact, it doesn't really fit into the current political spectrum at all.
On Liberty then is a defense of the individual against, first, the government, and second, the majority. It is also a quick and clear summary of what even non-philosophy folks know of JSM: the harm principle. Basically it says that people are free to do what they want so long as they do not harm the interests of another.
The ideas are very good, but it is the writing style that makes On Liberty one of those books you have to read. My yellowed edition is 118 pages long. It is a short book, but Mill is mathematically precise in his arguments. It is also important to note that Mill strongly believes in the power of argument. For Mill, if an idea is worth holding, it is worth defending. It is a refreshing contrast to the wilted political discourse we have today where different sides refuse to sit down and argue it out, and instead prefer to hide in their respective niches, talking to their allies only, and never engaging anyone who disagrees with them.
If you want a quick but dense read, pick up a copy of On Liberty. Tote it around as you make your way through the day, sitting and waiting in line at the DMV or some other place. It is worth the time.