1. Open the Play Books app.
2. Go to the My Library section. (To get to it, swipe the navigation bar out from the left side of the screen.)
3. Find the book you want for offline reading.
4. Tap on the menu button (3 vertical dots) under the book's cover.
5. In the pop up window tap on "Keep on device".
The book will be downloaded in what I believe is a proprietary format, so you'll need to view it using the Play Books app. To remove the offline version, follow the above steps, but tap "Don't keep" in the pop up window. The book will still remain available online.
You can't place a shortcut to a specific book on your home screen, but you can add the Play Books widget, which lets you easily flip through covers of all the books you have, then tap the one you want to read.