Can a book cipher be cracked without key? Book cipher works this way : 


Choose a reference book.
For each word find its position in the book.
replace each word by its coordinates (ie: 2,3,4 = page 2, 3rd line, 4th word).


How would somebody go about cracking a book cipher without the knowing what the key [or book used in this case] is? 

Finding the key is either impossible or would take a impractical amount of time.
Given a known plain text and a cipher text, finding the right key [or book] is impractical because all possible keys that match the plain text would require 
40 billion years using modern day technology to go through.

Example of a book cipher : Beale ciphers, it used the United States Declaration of Independence as a key.
First things first, finding the key (book) is not impossible, but just tough. If someone, like Google for example, has scanned millions of books into digital formats then it won't take long for them to figure out which book (simply try decrypting the first sentence only until the key is found, should be feasible for a mainframe).

Also, there is a lack of randomness. As the book you choose might not have random sequences the substitution will not decorrelate anything, which is another drawback. The best way to improve this is by choosing random references for each character by using a random number generator, which is tedious.