From Calibre's manual
http://manual.calibre-ebook.com/faq....ter-to-another
How do I move my calibre library from one computer to another?
Simply copy the calibre library folder from the old to the new computer. You can find out what the library folder is by clicking the calibre icon in the toolbar. The very first item is the path to the library folder. Now on the new computer, start calibre for the first time. It will run the Welcome Wizard asking you for the location of the calibre library. Point it to the previously copied folder. If the computer you are transferring to already has a calibre installation, then the Welcome wizard wont run. In that case, right-click the calibre icon in the tooolbar and point it to the newly copied directory. You will now have two calibre libraries on your computer and you can switch between them by clicking the calibre icon on the toolbar. Transferring your library in this manner preserver all your metadata, tags, custom columns, etc.
Note that if you are transferring between different types of computers (for example Windows to OS X) then after doing the above you should also right-click the calibre icon on the tool bar, select Library Maintenance and run the Check Library action. It will warn you about any problems in your library, which you should fix by hand.
Note
A calibre library is just a folder which contains all the book files and their metadata. All the metadata is stored in a single file called metadata.db, in the top level folder. If this file gets corrupted, you may see an empty list of books in calibre. In this case you can ask calibre to restore your books by doing a right-click on the calibre icon in the toolbar and selecting Library Maintenance->Restore database