A very interesting book, if an overly ambitious one.
Our hero, sitting on a hill in his home of England, disembodies and first explores his countryside, his planet, his sun, solar system, xenosolar stellar systems, foreign planets (here begging his joining with other minds), foreign galaxies, the entirety of space and time, foreign cosmos, and beyond.
It's heavily philosophical, to a point that I find it very difficult to understand. As a piece of science-fiction I find it marvellous. We have the first occurrence of such concepts as a Dyson Sphere (no, not named as such). It's beautifully written, and I've posted many quotes from it here, there were many, many more that I've skipped with unfortunance.
However as a philosophical piece I'm not as excited. He holds to a theory of panuniversal unity and community, that I have trouble getting into. It just feels a little forced. But the rest, just gorgeous. It's really something that you have to experience.
Now, there is a clear sense of warning. The inevitable crisis facing Stapledon writing this in 1937 quickly manifested itself as the Second World War, and you can see his fears growing within the pages.

You know that "Galaxy Song" in the Monty Python movie The Meaning of Life? Yea, if that you find humbling, you will love this book.

Another tip, and this is especially valid if you read at precisely the right speed; I read the beginning, where our hero leaves his body and explores the sun system while listening to the prelude to Tristan und Isolde. That's just nothing short of glorious.