[un_pogaz]

A realy quick way is to use the "auto-add from folder" feature.

Go to Preferences>Adding book>Automating adding and set a folder to target (need a restart)
After that, Calibre will be automaticly read and add any eBook in your target folder to the current Calibre library, then delete the original file from the folder. If your books is splited on several folder, move then to the target folder (your OS will be move file faster than Calibre read/add/delete the eBook).
Once you've added all your books, don't forget to disable the "auto-add" by clear the target folder field.

This solution are 2 major flaw: The book are added in mass without checking, so that can product a lot of wrong metadata that need to be correted AND the original book are permanently deleted by Calibre after being added in the library, so be undo available.
So that not the safer way, but probably the most faster.
The user interface of Calibre is so practical, convenient and efficient, that if your are well prepared, with a good metodologie, is can be a realy good solution.