Foundation and Empire (Foundation #4)
Isaac Asimov

I much preferred this to the previous book, especially the second half, called “The Mule.” In the original Foundation novel, there’s a feeling of inevitability, which is kind of the point. The fledgling Foundation faces crises, i.e. threats from other planetary civilations, but it always turns out that Hari Seldon predicted what was going to happen. When the crises are resolved, always in the Foundation’s favour, it’s revealed that it was the only way that events could have unfolded.
Foundation and Empire turns the idea on its head by introducing a character called “The Mule,” who conquers planets at an alarming rate, and threatens to take over the Foundation. Without spoiling too much, I’ll just say that the Mule is unique, and since Seldon’s predictions only work on broad social events, the Mule throws the preordained plan into chaos.
In contrast to the “everything always works out” ethos of the previous novel, the Mule creates real tension and suspense. We experience the story through characters who don’t know what’s going to happen next… which is basically like most stories we read, but feels refreshing in this world of predestination.