The Katha Upanishad or Kathopnishad is basically not a book but it is an exercept taken from Yajur Veda.
It is a story of a brahmin boy Nachiketa whose father Uddalaka, once conducted a ritual. In the end of the ritual he had to give some cows to the participant brahmins as a gift. Being a cunning man Uddalaka chose weak & sick cows to gift. Nachiketa ,when saw the shrewdness of his father, started nudging him whom he will gift his son to. In the beginning Uddalaka simply ignored his son's question but finally he got irritated and said to Nachiketa that, " I will gift you to Yamraj ( the god of death) ".
Nachiketa took this as an order and started on a journey to Yamlok, the home of Yamraj.
On reaching Yamlok, he found that Yamraj was out. Therefore he decided to wait outside. He waited for three days without eating anything.
Yamraj, when came back, found Nachiketa waiting. On asking, Nachiketa narrated him the whole story. Yamraj was impressed with Nachiketa's will and obedience. He granted three wishes to Nachiketa.
For the first wish Nachiketa asked that ,' let my father's anger be cooled down and when I go back he would love me as ever'. Yamraj agreed.
For the second wish Nachiketa asked that, ' fire is behind all the pleasures of heaven. This earth is also made up of fire. So please give me the knowledge of power'. And Yamraj agreed.
For the third wish Nachiketa asked Yamraj to let him know the secret of all secrets the brahma - gyan itself. This time Yamraj was shocked. He tried to persuade Nachiketa to ask for something else. But Nachiketa did not moved. In the end Yamraj had to give what Nachiketa asked for.
This whole Katha Upanishad is the dialogue between Nachiketa and Yamraj about the brahma-gyan. And the whole dialogue is immensely beautiful. There are some places where the reader gets confused but one shouls always keep in mind that the thing which Yamraj is talking about, cannot be said , but only can be experienced.
Some of the dialogues are:-
Yamraj says,'The brahma is universal and omni-potent. But it cannot be achieved by listening discourses or reading books but it is achieved only by those whom it accepts from it's own'.
Yamraj says,' If your senses are indulged in the outer world, you are going to miss it. For example your eyes sees the whole outer world but does not sees it in your heart. Your ears hear everyhting but misses the sound of it.
Yamraj says,' One who knows this secret , will not speak. But he will be silent. And he spontaeously resides in the kingdom of it.
The author of this upanishad is unknown. By the way there are atleast 108 upnishads said to have been written nearly around 1500-600 B.C.
The word upnishad simply means to sit near the GURU and listen.
This book, I love a lot.