Books are single-click/tap only; they always open when clicked or tapped. This means that if you already have your hand on the cursor, you can just click on the book you want to open. If you are using a touchscreen device, you can tap the book you want. If you are on the desktop navigating with the keyboard and have actually managed to get a book selected but not opened, it is faster to use the return or enter key to open the selected book. There is no use case where clicking on the "Open Selected Book" button is faster than your current input device's primary method of opening things.