The length of a book depends upon what type of book it is and what publishing route you decide to take. If you plan to sell your work as an e-book, you can have as many or few pages as you like. You can probably do the layout yourself in Word and then convert it to a PDF. If you want to have your book printed, there are other things to consider such as the genre, weight, size, and price point of the book.
A center-stapled chapbook (quite popular with poetry, but difficult to lay out) can be printed on your home printer if you only need a few copies. If you plan to market the book, it would be more cost effective to publish it on demand with a service such as Published by Westview. In that case, you will get more bang for your buck by adding more poems and filling the book to about 100 pages.
Shipping costs are dependent upon the size and weight of the book. Weight can be cut by using thinner, uncoated paper; such is the case with mass market paperback novels. Even though they are thicker than a standard 6x9 book, six and a half by four-inch novels weigh about the same or less than their counterparts. That’s because the typical paper used for nonfiction book interiors is a 60# smooth stock, while mass-produced paperbacks are printed off-set rather than digitally. Off-set (high quantity) printing costs about 80 percent less than digital printing that produces one book at a time.
For a perfect bound (glued spine) book, you should also consider your genre. I highly recommend a strolling research through your local bookstore to see what size and page count your genre is currently averaging. A nonfiction book usually has a word count of about 82,000, which equals about 300–350 pages when printed to 6x9 standard size. This number is based on 275 words per page using 12-point type. A nonfiction book with 350 pages will have a spine width of about one-half to three-quarters of an inch.
Since novels tend to have more pages than nonfiction works, it’s not uncommon for a novel to have 500-600 pages. The font or type size in a paperback novel will be much smaller than the average self-help or how-to book. That means more words per page.
Also note the retail price of the books in your genre. You want to price your book within the genre price range in order to sell your book. If you have to mark the book up in order to make a profit, you would do well to reduce the page count and thus, the print cost. This can be achieved by using a smaller font.
Remember, books layout in blocks of four. If your book has an odd number of pages, you will undoubtedly have some blank sheets at the end. Hint: use these extra pages to market your other books or services.