In Adobe Lightroom 3, how can I reset the library? That is, I want to remove all photos from the library.
The catalog is an *.lrcat file. If you delete it will be gone. The next time you start Lightroom tell you it cannot find the catalog and will create a new one.
There is also usually a folder called Backups next to it which contains backups of your catalog. You will also find a folder called *.previews.lrdata which is a cache which you can delete anytime, even if you do not delete the catalog. That contains previews which are created as needed.