There are at least three ways to transfer files over USB. Manufacturers and users confuse and conflate these with inexact terminology.
UMS - USB Mass Storage
This is when a device pretends it is a flash drive. In Windows it gets a drive letter (like G: ). Your device usually can not operate normally and the screen will show a message, "Connected to computer". You can copy a file easily with "copy mobydick.epub G:\". Using this will
always require that you "safely disconnect". Corruption of the file system is always possible.
MTP - Media Transfer Protocol
This is a variant of a protocol first used for digital cameras (PTP). Windows (File) Explorer understand its and will show a tree with your device's name in the left pane. You cannot easily transfer any files except with drag-n-drop. Regular desktop applications can not access any device files unless already transferred.
ADB - Android Debug Bridge
ADB is like a superhighway between your device and your desktop. It can carry multiple traffic for different purposes. One of these is transferring files. You can copy a file easily with "adb push mobydick.epub sdcard/Books". Like MTP, regular desktop applications can not directly access device files, but unlike MTP, ADB can easily be automated to transfer or sync files.
I use my own adbsync.exe utility (in my sig) to keep my books updated on multiple devices.
Code:
C:\>adbsync /acsu /s C:\Books sdcard/Books