Book cipher is a kind of secret code, that uses a very common article (a book) as the key. All they have to do is to transmit the location codes that are needed to pinpoint specific words in that book.
If you don't know which book, no way of cracking!
Choosing the book needs to be very specific... it must be the same title AND print. In another print of the same title, the words and lines may be on different pages! Also, if you plan to transmit sentences, you need to choose a book that holds all the words that you need for your messages.
For this very reason, the book of choice is often a dictionary.
A dictionary holds so many words, that it won't stump you that easily!
Here at TopSpySecrets.com, we often use the 'How to become a real spy' booklet, as most agents here have downloaded it as they subscribed to Messages from U.N.C.L.E. Haven't got it already? I'm sure you can find it with your micro dot spotter.