There's not a specific book where this information is laid out. I'll write what I know and can refer to source if you have questions. At the outset, I'd like to mention that there are dualistic (dvaita), non-dualistic(advaita) and qualified non-dualistic (vishishta-advaita) interpretations of the Upanishads. As a follower of advaita, I clearly see from texts that non-duality is what these scriptures teach.

The Mandukya Upanishad talks about "ajata-vada", ie the argument that nothing is ever born. Its teaching is very close to buddhist philosophy.

The Mundaka Upanishad talks about the creation being God itself. A famous mantra quotes an example of a spider which is not only the efficient but also the material cause of the webs that it spins.

Chandogya Upanishad chapter 6 has the "maha-vakya" i.e. great statement "tat-tvam-asi" - thou art that.

Of course, reading a translation will not be sufficient to understand the depth of this text. These texts are usually expounded by scholars who are steeped in the tradition of studying and understanding them. For advaita (non-dualistic) philosphy, the master-commentator Shankaracharya has written brief commentaries on all the Upanishads. Their translations are available in English, but like I said earlier, a mere reading of them may not bring about the desired affect of "oh my god this is so awesome".