Restoring a Library Backup, and choosing only to restore the Library portion, will transfer everything that is part of the Library (imported files, Media Views, Playlists and Smartlists, and other similar things).
Restoring a Library Backup, and choosing to restore the Settings portion, will transfer
everything else of relevance (primarily things set in Tools > Options and the DSP Settings). This would include user-defined fields.
This is covered in the Library Backup article:
http://wiki.jriver.com/index.php/Library_BackupThe simplest way to migrate things is to restore a Library Backup. There is not a solution to "merge" Libraries. There are some things you can do fairly quickly, manually with copy and paste, or via creative Settings file edits (though the latter is not supported) if absolutely necessary.
But, this is why I usually strongly recommend to never make separate Libraries unless you really,
really,
really won't
ever need anything from one in the other. Almost every time I've ever seen this kind of request, is because the user should not have been using separate Libraries in the first place. If you find you regularly need to migrate things, intact, from one Library to another, it is an extremely strong clue that you're using multiple Libraries in MC in an ill-advised way.
The most common answer I see to this is "I wanted to keep the media in Library A segregated from the media in Library B" (because, perhaps, one set is Pop music and the other is Classical, or something like that). This is
not a good use for multiple Libraries. That's what Media Views are for, and it works much, much better (and has the added benefit that you don't need to restart MC to switch from one to the other).