BCID means BookCrossingIdentity, and it's the number a book registered with bookcrossing.com has. With this number its itinerary around the world can be followed, and if people are doing journal entries for it, you can also see where it has been and who has read it and so on.
Bookcrossing is free, non-commerical and international, bringing together book lovers all around the world. If you're interested, don't let you be discouraged by the website - it's neither very practical nor very logical, but the fans are defending it with a lot of love. *g*
Now, this book, A Time of Gifts by Patrick Leigh Fermor, is a special one for me, so I chose a copy of it to register it as my 300th Bookcrossing book. Here is its journal entry including info about the book and why it is so special to me.
Bookcrossers usually release their books into the wild, so everybody can read them, because he basic idea is to make the world a library for everybody. But also sometimes books are simply exchanged, or send on a book ring, which means everybody who signs up for it gets the book in due time, reads it, and sends it on.
I started an international book ring for this book and the second volume to it, and it has travelled literally around half the world, from Germany through some European countries, to South Africa, the U.S.A., Canada (Newfoundland, to be precise), and back to me.