It's pretty easy to copy multiple ebooks to your iPhone using iTunes. If you're using iBooks, you just have to drop the books in the books part of itunes I think. But since you seem to be using other apps, you'll have to open the apps tab on your device, then scroll down to the file sharing area. You can then drop your files in the file sharing area of your app. Here's a YouTube video that should give you an idea what to look for (the hardest part is to locate the file sharing area, after that it should be easy).
https://m.youtube.com/watch?v=eQ-LcjC7Zww