This is one of the three best books by Terry Pratchett, at least according to my personal taste (the other two being "Small Gods" and "Hogfather"). It is very unfrequent to find so many qualities in one book. All Discworld novels are funny, but this one is purely irresistible.
The gallery of exceptionally funny characters is simply unbelievable. This is the first of books about the Night Watch in Ankh Morpork so be prepared to meet many new characters - Sam Vimes, hard drinking police veteran, very low on his luck but destined to a great future; sergeant Colon, possibly the best caricature of the typical Bobby I ever saw; corporal Nobby Nobbs, who, as we all know, was "disqualified from human race for shoving"; and especially constable Carrot, the biggest dwarf who ever lived and possibly the ONLY honest person in all Ankh Morpork.
Those four guys form one of the most likeable cop teams I ever saw - and they have a really big problem to deal with... the kind of problem that sleeps on golden hoard, breaths fire and eat maidens. Then there is also the secret society - possibly the most hilarious secret society I ever saw in a book, plotting to take power and never paying for pizza... And of course the Patrician or Lord Havelock Vetinari, the smartest, the most devious and most dangerous (although not TOTALLY evil or tyranical) ruler in the Discworld history.
This book is a delight. There is not one weak spot, not one failed dialogue, not one weakness in the plot - only a continous, smart, totally comical story about a political plot gone horribly wrong, a small police squad desperately trying not to get eaten, a city that gets a new king and a GREAT (or maybe it is small) love story which will need an old copper tea kettle to find a satisfactory conclusion, the item being eaten in the processus...
Buy it and read it! You will not regret it and you will be then able to enjoy other Night Watch stories in Discworld novels - they are worth it.