This is a valuable document that is the translation of 30 minor upanishads. Now even though they are called "minor", they are priceless and a sincere seeker is well advised to get a copy (free) and read, contemplate and meditate on them.
https://ia800303.us.archive.org/10/items/thirtyminorupani00xxxxuoft/thirtyminorupani00xxxxuoft.pdf