Project Gutenberg has tons of free books available for download. Unfortunately, the formating leaves much to be desired when they are viewed on the Kindle.  Gutenmark to the rescue. This neat commandline utility can reformat the Gutenberg book into a much more readable html format that Kindle displays nicely.

To remove the custom screensaver pictures you added to your Kindle, do the following;

1. Connect the Kindle to your PC with the USB cable.
2. Browse to the ‘system’ folder on the Kindle.
3. Open the ‘screen_saver’ folder that is in the ‘system’ folder.  If the folder is not visible, set the folder preferences in Windows Explorer to “Show Hidden Files”  and uncheck the “Hide Protected System Files” option. To do this, click Tools from the top navigation menu, then select “Folder Options…” Make the changes, then save and exit the menu.
4. Delete the .gz files and the screen_saver_last files in that folder.

You may see a cached leftover screensaver picture the next time you hibernate the Kindle, but it will then start to cycle through the default ones that came with the Kindle.

For the technically inclined, there is a great series of articles by Igor Skochinsky detailing his efforts to reverse-engineer the Kindle. He explains how he gained console access to the underlying Linux OS and explores the features (many undocumented) and workings of the Kindle.