You Only Live Twice has the feeling of a grand finale. It's not, of course, but it feels like one. Bond is suffering depression following the events of On Her Majesty's Secret Service, and his work is suffering. M decides to set him an impossible mission to snap him out of it, and off Bond flies to Japan.
The book is neatly split into two halves along Fleming's usual lines. The first is about Japan, and Bond's assimilation of a new culture. This is typical Fleming mix of fact and fantasy that nevertheless convinces the reader that he knows the area, and successfully adds that touch of the exotic that Bond stories are known for.
The second half is the action part of the story - more similar to elements of the films (which of course by this point had begun) and rounds things off nicely.
One of my favourite things about this book is Fleming's sense of humour. The opening chapter is almost entirely taking the mickey out of one of the repeated elements of the series - following the classic card games of Casino Royale and Moonraker. Later he makes reference to Hollywood, and even breaks the fourth wall with a little dig at himself.
It's another excellent adventure for Bond. He has far more depth than the screen portrayals suggest and is one of the richest characters in fiction, despite the time that has passed since the stories were written. I've really enjoyed re-reading the series and look forward to a time in fifteen years when my memories have faded and I can enjoy them once again.