Some books are meant to be read together.
It’s seriously important for historical fiction, when you want different perspectives: I recently read Johnny Tremain and My Brother Sam is Dead back to back–one celebrating the fight for freedom, the other showing the brutality of both sides and vehemently anti-war. I got a lot more out of them by reading both together. Come to think of it, I should re-read Chains and Forge to round out the story (Tremain’s attitude towards anyone who’s not white and male is problematic, to say the least).
Some authors do a great job of complementing their own works. Karen Cushman is my go-to guide for medieval Europe: Catherine, Called Birdy for manor life, The Midwife’s Apprentice for villagers and Alchemy and Meggy Swann for life in the city.
And then there are the books that should be paired up just for fun: