You can transfer E-books from a computer to a Kindle device through a USB cable. When your Kindle device is plugged into your computer, it will appear as a removable mass-storage device. When you click on it, you will see several folders. One of the folders is named “documents”.
Go to www.amazon.com/manageyourkindle and log in with your Amazon E-mail and password. Locate the E-book you want to transfer to your Kindle. Click on the “Actions” button and select “Download & Transfer Via USB”. At “Which Kindle will you transfer this to?” choose the Kindle that you have connected to your computer from the drop-down menu. Click on “Download.” You may be prompted to “Save” or “Open”. Choose “Save” and select a place to save it where you can easily find the E-book file, such as the Desktop. Move the file into the “documents” folder of your Kindle. You can move the file by dragging and dropping.
Eject your Kindle and unplug the USB cable. Your E-book will appear on your Kindle’s home screen.
Amazon has a video that illustrates this process: http://www.amazon.com/gp/mpd/permalink/m1VARB8H29L2XP/ref=ent_fb_link
Amazon’s video focuses on purchased content (not borrowed library E-books). While the process is similar, you can disregard the parts about purchasing content.