For last Saturday's read-a-thon, I finally read three slow cooker cookbooks I borrowed from Lady K. Not only will we get to share conversations about the recipes, but I was treated to some awesome marginalia. I loved seeing what she thought of specific recipes as I was reading.
In addition to fun marginalia, borrowing books from friends comes with additional insights. You can talk to your friend about what they liked and disliked about a book. You can wax poetic together about shared frustrations or loves with the the text. If the book is from a series, now you have someone to wait it out with until the next book is released. If you both had different opinions about the book, you can have spirited conversations as to why.
Furthermore, books shared among friends tend to be lent multiple times. When books travel amongst friend groups, it creates a casual book club. You might not all be reading the same book at the same time but, sooner or later, you can end up at a party together all talking about the same book.