Yes, same here, need the processing, but that doesn't stop me to start a new book meanwhile. While doing the dishes or before going to sleep or while doing my groceries or riding my bike, I process the book I still need to think about. A few years ago I started a reading journal. In it, I keep a note of everything I read, if a book is special and why, favourite lines from the book, sometimes how I found it too. While writing about it, if needed I analyse it, think about it more and that helps.