Outside of Samsung's yearly announcement of a new Galaxy Note, it's rare to see anything interesting happening with a stylus. However, the latest update to Google Play Books comes with some new behavior that makes a stylus much more useful with things like college textbooks and other reference material. Now when you drag a stylus over some text, it instantly switches into selection mode and pops up the action menu as soon as the selection is done.
In previous versions of Books, a stylus worked just like a finger. Swiping left or right would flip to the next or previous page, and you had to long-press on a word to switch into selection mode. Once in selection mode, you would have to drag handles around to expand to a larger passage. It was fairly tedious and a bit slow.
With Play Books v3.15, dragging a stylus across text will automatically begin selecting text as you drag–no need to long-press or mess with handles. After lifting up on the stylus, the standard action menu will appear. If you choose to highlight the text, the stylus will switch to another mode that automatically highlights new selections. As always, tapping on a piece of highlighted text will open the action menu.
The other gestures remain unchanged with a stylus, so you can still flip through pages by tapping on either the left or right side of the book, and tapping on any non-highlighted text in the middle of the page will bring up the overview mode.
Most people aren't adding highlights or making notes in novels or comic books, but those things are pretty common activities in textbooks, cookbooks, how-to guides, and other reference materials. Students will probably get a lot of use out of this since highlights and notes are also automatically stored in the Notes screen for easy reference (open the overview, tap the "contents" button, and move to the Notes tab).
The APK is signed by Google and upgrades your existing app. The cryptographic signature guarantees that the file is safe to install and was not tampered with in any way. Rather than wait for Google to push this download to your devices, which can take days, download and install it just like any other APK.