When you are plotting the layout of your book, one of the first questions you need to answer is what physical size the book should be. Go into any bookstore and you'll see that books come in a wide variety of sizes and shapes. So what is the "right" size for your book? The answer depends on a number of variables.
1. The genre. Some types of books are invariably a certain size. For example, many mass-market paperbacks are are small (less than 5 inches wide and 8 inches tall). Many computer and technical books are larger, such as 7.25" x 9.25" or thereabouts. If there is a standard, it's wise not to deviate, particularly if you plan to sell in book or retail stores, since display racks are often set up to work with a specific size book.
2. Your printer. Book printers have standard sizes that they print. When you are getting quotes, you need to make sure you are comparing "apples to apples." Start researching what your printer's standard page sizes are before you do any layout work. It can be difficult to visualize the difference between 6" x 9" and 7" x 10", so head to your bookshelves and start measuring the books you have. Also if you will be printing your books using both print on demand and offset technology, make sure you select a size that can be printed either way. Printers often offer fewer sizes for color or hardback books, so keep that in mind as well.
3. The content. A smaller physical size generally means you will have more pages in the book. With a 50,000-word book, a 6" x 9" version might end up being 200 pages, but only 170 if you lay it out in 7" x 10" format. If you are using print-on-demand technology, the pricing depends on the base unit cost and a per-page charge. Different pages size can incur different unit costs. For example, Lightning Source has two different categories of paperbacks: small and large (each of which includes multiple sizes). Not surprisingly, the base unit cost is more for a large paperback. If you can't decide between a couple of page sizes, "run some numbers" to determine which option will result in a lower print cost.
4. The layout. If you are including diagrams or photos in your book, you may need to select a larger size to accommodate them. No one likes squinting at tiny diagrams. When it comes to books, readability is paramount. So make sure your layout allows for generous white space and margins. If necessary, do a preliminary "dump" of the raw text into a couple of different layout sizes and see how long the book ends up and how much white space you need.
5. Your audience. If your readers are older, you may want to use a larger font size, which makes the book longer. In this case, you might consider using a larger physical size to keep the page count down.
Okay, now that you have some food for thought, here's a real-world example. Our book Web Business Success is basically a technology book and includes a few diagrams. We opted for the 7.5" x 9.25" size because on our shelves we found that is a common size for computer-related books. The size still falls into LSI's "small paperback" category so the base unit cost is lower than it is for a 7" x 10" (another common size for computer books). The larger format gave the room we needed for diagrams and kept the page count down, so the cost per book is less than if we had opted for a smaller size.
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