I found this book on Project Gutenberg a long time ago, but can't find it now. It's a pretty old book. The premise was that there are these four people, two men and two women, who build a spaceship and go explore space in it. The ship had to be controlled by human minds (telepathy/telekinesis are prevalent in this society), but they didn't figure out until the middle of the book that they could actually determine where it went. They interact with a lot of Earth-type worlds with human-ish inhabitants, and theorize that the universe is really just a gigantic organism (they encounter something that's analogous to cancer).
I remember one of the women had green clothes and hair (and maybe skin? I can't remember), and the other was nicknamed "Brownie." Don't laugh, but one of the premises of the book was that all these people who have telepathy and/or telekinesis also just happen to be gorgeously attractive.
Anyone know what book this is? I really wish I could find it. Weirdest book ever, but it was a fun read.