The word count is how many words are in a document or a book, or even a paragraph or line. Usually, when teacher talks about word count, he/she is talking about minimum length requirements when you are writing a paper. When editors talk about word count, they are telling potential authors to keep their articles and books within a specific length.
Most teachers would not require you to know the word count of an entire book unless there was a purpose for it. For example, are you discussing whether or not this book is categorized as a short story or novel? Short stories are under 7,500 words, a novelette is 7500 to 17,500 words, and novellas are 17,500 to 40,000 words, while a novel is usually over 40,000 words.
If what you want to know is the word count for the book itself, publishers use approximate word count based on range of between 250 to 500 words per page. The Bronze Bow has approximately 10 words per line and approximately 31 lines of full type (approximately 2 to 3 partial lines) for an approximate total of 310 words per page (using the Houghton Mifflin edition available from Amazon.com): 10 words per line x 31 lines per page = 310 words per page, approximately.
To get the approximate word count total for the book, multiply 310 times the total number of pages in the book. This will give you the approximate word count the publisher uses when choosing and planning manuscripts to publish.