Getting An Exact Count
Examine a book word-count directory. This method is necessary if you need to achieve an exact word count and is doable if the book for which you need a word count is well known. There are multiple online resources that publish directories of well-known books with their corresponding word counts. These include the Renaissance Learning online bookstore, Amazon's "Inside This Book" feature, various blogs and potentially book publishers who may choose to publish information and statistics about their books.
Search for the book using the directory's search interface.
Find the word count feature, which will vary by directory. For instance, Amazon places the word count inside the "inside this book" option while Renaissance Learning places the word count near the top of the book's description.
Estimating Based on Industry Standards
Determine the industry standard of words per page for your book. For example, in most cases it is considered to be an industry standard for books such as novels to contain 250 words and 25 lines per page.
Multiply the industry standard by the number of pages contained in your book. If the industry standard is 250 words per page and the book you read contains 200 pages then you could reasonably expect that the number of words read is 50,000.
Ensure all pages were included in the count and adjust the count if necessary. Consider that some if not many publications may not number the book's preface. Add the words on these pages to your count.
Sampling for Author's Writing Style
Count the number of pages in the book. Sub-divide the count into full, 1/2 and 1/3 pages. If necessary, round the count to the nearest of the three size specifications.
Generate the overall page count by multiplying the number of each category by the category size and adding up those three numbers. For example, a book may have 155 full-pages (totaling 155) , 18 1/2-pages (totaling nine) and nine 1/3-pages (totaling three). Your total count in this case would be 167.
Discover the average number of words on a page by counting the number of words on three randomly picked full pages distributed throughout the book. Average the count by taking the total number of words counted and divide by three. This should capture the authors writing style and provide a reliable estimate of the number of words used by the author on each full page.
Multiply the average words per page by the overall page count. This should provide a close estimate of the number of words in the book.