The FBReader
web site, includes
screenshots of FBReader on the Nokia 770 which also server as a mini-tutorial. Note that when you click on the images you get full (800x480) size versions. These will most likely appear bigger on your desktop screen than on the device, because the 770 has about 220 pixels per inch.
One of the advantages of FBReader over most e-book readers is that it supports a wide range of input formats. This means that you can usually find existing files that will work "as is" on the 770. It does not support PDFs, because they are not "reflowable" (i.e. they can't be reformatted to fit the screen). There is a separate PDF viewer for the 770/N800, but the lack of reflowability makes PDFs problematic on the small screen.