The Truth is Terry Pratchett's landmark 25th book in the Discworld series, and a very, very fine book it is. It's designed with plenty of appeal for longtime fans and new readers alike, plus the usual assortment of puns, historical allusions, digs at pop culture, and sarcasm that fans of Pratchett (not to mention Douglas Adams of Hitchhiker's Guide fame) have come to expect and love.
The book takes place in the well-established Discworld city of Ankh-Morpork, a parody of London with a few sprinklings of New York City thrown in. Ankh-Morpork is governed by the Patrician, a man who doesn't rule so much as check and balance the cities business and criminal interests against each other, and policed by the Night Watch, who have had several Discworld books to themselves but this time ride along as guest stars. The main character is William de Worde, a young man who comes from a family of aristocrats but lacks the wealth and influence of aristocracy himself.
William has eeked out a living up until now by providing a monthly newsletter to foreign dignitaries regarding the major events in Ankh-Morpork society and politics. Ankh-Morpork has a Guild of Engravers who painstakingly produce metal plates of his letters in order to mass-produce several copies, but William is soon given the means and the motive to increase production when a team of dwarves smuggle a printing press into the city. (The Guild of Engravers had convinced the Patrician to make printing presses illegal within the city before now.)
That's the means. The motive is an apparent murder by the Patrician on one of his own employees, and the Night Watch are unable to get to the bottom of it. It's up to William and his rapidly-evolving printing press to find out what's happening while the Watch, the Engravers, and assorted criminal elements try to (literally) dig his new newspaper out from under him.
William de Worde is a guy who's just trying to get to the truth, the real truth, while everyone around him is telling him the "real" truth just isn't important. Only the Patrician seems to be on his side, and with that man implicated in a murder, William's support is constantly unravelling around him. Even his own employees are caught between, alternately, their families, their heritage, or their need to make money and their desire to help William keep the paper alive.
Every Terry Pratchett fan has his or her own sliding scale as to what makes a book good or bad -- more jokes, less pop culture, more characterization, less mythology, more history, less sarcasm, more established characters, less unnecessary characters, and so on and so on. So far, The Truth seems to rate highly on just about every fan's meter due to a healthy blend of all of the above. The Ankh-Morpork Times encounters reformed vampires, tabloid competitors, Discworld photography (a little demon in a box paints whatever he sees, and magical eels provide the flash), family royalty, gangsters in black suits who keep saying "____ing", journalistic ethics, and of course the omnipresent Death (tall, black cloak, has a thing for farming implements). Those who understand the cultural and historical parody will laugh twice as hard, and the rest will enjoy rereading it years later and picking up on jokes they didn't understand the first time.
The Discworld series began years ago with The Colour of Magic and The Light Fantastic, but a lot has happened to Ankh-Morpork since those days of mere sword-and-sorcery parody. The Truth is a good point to start if you're a new fan, and should be enough incentive to go back and get the rest.