Doors of Eden
Adrian Tchaikovsky's 2020 novel. Two cryptid hunters go missing on Bodmin Moor and one returns four years later, which turns out to involve parallel Earths where evolution took different branches, including one where trilobites became the dominant intelligence. The narrative alternates with extracts from an invented natural history of those branches. It is a big-idea science fiction novel with a spy-thriller chase running through it, and the field guide sections are the best part.