Fifth Planet   by Fred Hoyle, Geoffrey Hoyle

This is probably the purest science fiction book I've ever read, every aspect being speculative. It is however extremely dry and comes across more expository than literary, and some of the human reactions to events could have done with some exploration.

It is a book of two halves. The first part is an ultra-realistic depiction of a journey to a world of a star system passing through our own, under the pretence that a star is slowly passing ours and takes years to complete the transit; unlikely?

Second part even more speculative, where an alien life force has managed to make the journey back to Earth, occupies the body of an associate of one of the original astronauts, and manages to give everybody on Earth the illusion that a global nuclear war has started. In real life, this would almost certainly have led to a real war and nuclear holocaust?

But suspend your disbelief about the two massive unrealisms, and the rest is a really engaging piece of speculative fiction: incredibly intelligent and the contrast between Fred (professional astrophysicist) and Geoffrey Hoyle (his son) comes through beautifully.