Originally, Google Books was only good at doing one thing: reading content from the Google Play Store. This meant you could read free books, or the ones you purchased -- and that was it. When the update allowing your personal files to be uploaded to Google Books came along, the app made a giant leap forward. Now Android devices could be even more competitive with traditional e-readers that allow side-loading.
Unfortunately, the upload feature was limited to your Web browser. This meant navigating to the Play Books Web site on a computer, or within the mobile browser, to upload your content. Clearly, there was a disconnect in the process.
With the most recent update to the Play Books app, the ability to upload PDF and EPUB files directly from Gmail or the Downloads folder has been added. However, you will need a copy of the latest Play Books app on your device. If you haven't received the update, you can install a copy from here, provided by AndroidPolice.
After you download the APK, locate it in your Download folder and manually install it. You will need to have the unknown sources option enabled in Settings & Security.
Now that you're set with the newest version, here's how to use the upload feature:
First, locate a PDF or EPUB file in your Gmail messages or Downloads folder. The Downloads folder can generally be accessed through the file explorer on your device. For instance, it's called on My Files on most Samsung devices.
Second, tap the file once to open or preview it, and there will be an Upload to Play Books option in the pop-up menu.
That's it! Now you can easily add more content to your Google Books without needing to be in front of the computer.