I read this book (first) 15 years ago and it is still dear to me; it helped to bring about a breakthrough. I was then just getting acquainted with Hindu tradition, its mythology, philosophy, mysticism. I had felt existentially out of tune, depraved, and basically alienated for quite some years, and with this book I finally felt `they (the Hindus) know how to face and handle my deeper existential needs (which to me are not quite clear)'. I felt `now I am about to get to know who or what I really am'. I was 24 then. I was quite grateful.
This book offers a very useful introduction, providing context and understanding, and a selection of the main Upanishads. It is a good introduction. For further study, of the complete translations I recommend `The principal Upanishads' translated by Sarvepalli Radhakrishnan.