A wonderful book; very much worth reading, particularly the middle section. I believe that this book could have been quite successful as a short story encompassing only that section. While most aliens are portrayed as impervious, disgusting or frightening, Asimov's are simply fascinating. There are two distinct types -- the "Soft Ones" and the "Hard Ones". Members of the former are divided into three sub-groups: "Parentals", "Emotionals" and "Rationals". These beings join together to form "triads", composed of one member of each sub-group. Only as a threesome can they "melt" together, combining their atoms together into one, thicker version of themselves.

The book is strangely disjunctive -- the other two sections are more typically Asimov, about human scientists, devices and politics. But the surreal, beautiful portrayal of the aliens and the parallel universe is something I haven't forgotten...