They each read different books. Not just different - of different kinds.
She was a lover of fiction; he of non-fiction. She always had at least one novel on the go - sometimes more. He was reading, or re-reading, or referring to, works of philosophy, theology and psychology, to inform and guide his own thinking and experience.
He was typically reading aloud from his books, enthusing, seeking to share the insights he had gained, or thought he had gained, and relate them to people and circumstances they knew. She was more often lost in her books - lost to him - as if for now in a closed and complete and separate world - self-contained, set at a safe distance from duty and the mundane.
He read books as he had learned to do bible study in church groups - seeking wisdom, guidance, searching for meaning and new ways of being - listening for a voice, a message that would change his life for the better, show him who he was and who he might become.
She read, and was charmed, amused, entertained, distracted, educated and informed. She read to increase the circle of her friends - those fictional characters who populated her imaginative landscape. This was friendship for its own sake. She was happy to be with them, and to reflect upon the nature of that being. No hidden purpose or ulterior motive came between, or behind, them.
See them on a quiet afternoon. Two people. Two open books.