One of my all-time favorite books by my favorite author. The first two sections are outstanding time-travelling, dimension-hopping Science Fiction. Sadly, the third section falls to Heinlein's efforts to tie all of his books into one story line.
Excellent entertainment, better than even 'Stranger in a Strange Land' (also by Heinlein), this book is on my best-ten-fiction list. Vivid characters, good science, great plot. I will keep it and re-read it in a couple years.