The Upanishads, the Bhagavad Gita and the Brahma Sutras are all about Vedanta, including Advaita (Vedanta). In fact, they are the three main sources about Vedanta, known as the Prasthanatrayi:
I have not read Radhakrishnan's take on the Bhagavad Gita fully. I only read parts of it, when I was a kid. We had many books in that category (Indian spirituality and mythology) lying around at home. So I read some of them.
But I have read one or two other versions of the Bhagavad Gita fully, later on, but don't remember who they were by. There are some pocket editions of the Gita available too. I used to have one. If you are into that subject, I recommend getting a pocket one (along with a bigger one), so that you can read it when you are on the go.
As for Eknath Easwaran's books, I have only read these two: Meditation and The Dhammapada. So I cannot specifically recommend any of his other books. But they are likely to be good, based on his Wikipedia page.