A breakdown of the Upanishads by Parthasarthy. Mostly a good analysis of the texts and explains the concepts well.
But when it comes to some of the more difficult Upanishads, specifically covering non-dualism, Parthasarthy gives an unnecessary flowery and poetic explanation in a way that wouldn't be helpful for people new to the concepts. Generally good just because the Upanishads themselves have a lot of wisdom within them.