There are many Upanishad
Upa means near
Shad sitting near
The wisdom that is obtained by sitting near a Guru.
There are 108
Well to know the Upanishad you have to sit at the feet of an enlightened Guru. There is no other way
So in the name itself it tells you everything
So the next question is what will I gain by sitting at his feet?
So that is what is written in the Upanishad.
For example in taittriya Upanishad there is detailed description of Dahar vidya.
Many Upanishad describes Om in detail
Others talk about atma
Some are in parables others in stories
Well if you read them you will have a description of say different samadhi Now you know what samadhi is.
But have you experienced it ? Obviously reading the book or pretend mediation can’t give it. It is a state that is slowly reached at the feet of a Guru. Hence it is Upanishad
I have enclosed a link to one great Upanishad. Krishna Yajurvediya Upanishad
https://www.wisdomlib.org/hinduism/book/varaha-upanishad-of-krishna-yajurveda
It caught my attention because surprisingly in one shloka it gave the ultimate trick. Lahiri mahashaya commented on this book too with his commentary of many of the shlokas. It is a collection called Yoga Niketan
The greatness of Hinduism is highlighted in these books Truly a marvel
Bhavin