Counting words is a sort of Black Art: What is a Word?
This script will open all of the documents in your book, count the words according to 2 different methods (I'm sure other scripters will find more ways), then display the results. Then choose which number to believe 
//DESCRIPTION:Count Words Per Book
// A Jongware Script 18-Aug-2010
book = app.activeBook;
words_a = 0;
words_b = 0;
for (var d=0; d<book.bookContents.length; d++)
{
doc = app.open (book.bookContents[d].fullName);
words_a += countWords_a (doc);
words_b += countWords_b (doc);
doc.close(SaveOptions.NO);
}
alert (“Number of words: “+words_a+”rOr possibly: “+words_b);
function countWords_a (aDoc)
{
var w = aDoc.stories.everyItem().words.length;
return w;
}
function countWords_b (aDoc)
{
app.findGrepPreferences = null;
app.findGrepPreferences.findWhat = “\w+”;
var w = aDoc.findGrep().length;
return w;
}