Originally Posted by Tolkamp
A friend of my has a Calibre library with books.
Is it possible to add (merge) this library to my own Calibre library?
Yes. Copy his library to your computer, connect to it using the black triangle to the right of the library icon, then select all of the books you want merged and right click, choosing Copy to Library and send them to your library. You can then connect back to your library and delete his library, if desired.
Before doing this, decide if you want duplicate books automatically merged by author/title matching. If so, turn on Automerge in Import/Export|Adding Books. If you want to get all of his books in your library, and manually merge any duplicates (using the Duplicate Finder plugin) turn off AutoMerge.