Upanishad.

There are 108 Upanishads. 9 upnishad from Gita press and from Satyananda. 45 Upanishads. Then there are some from arbunido and all.

9 upnishad from satynadana seems to be good priciple book to start from. And accompany this by 108 upnishad and 9 Gita press upnishads. This should give you right meaning and then you can search and go deep in each one.

The process of reading these upnishad is to go through one. Do chittanupasna and then do the second and so on. There is no hurry. It is ok if you do not understand the meaning of some passages.

Order these 3 books.