This has happened to me several times - I'll be on my phone and really want to share a link to a book. I'll look over the entire book page a couple times and get frustrated that there's no option to share. Then I'll start to wonder why there's an upload icon in the upper right. What would I be uploading? Upon tapping it, I discover that it's meant to be a share icon. I've only ever seen that icon used for uploads, and most apps are consistent with using an icon that looks like the Material Design share icon for sharing, so that's what I look for.