With the e-book, paperback and hardcover editions of my book "Green Technology Strategies" available, I thought I might try a Large Print edition. These books use fonts of 16 to 20 points to make them easier to read for those with limited vision.
Guidelines usually suggest a sans serif font and wider spacing for large print books. For a novel, the larger font is usually accommodated by using smaller margins, a slightly larger page and increasing the number of pages in the book. But for a textbook, the page numbering can be significant.
The largest paper size offered by LuLu's print on demand service is A4 (8.26 x 11.69 inches). My PDF typeset original is designed for U.S. Trade (6 x 9 inch) with a 11 point Times Roman font. Simply by printing on larger A4 paper will enlarge the pages 130%, increasing the font to 14 points. This is a bit small for a for a large print book. I could reduce the margins to .5 inches (the minimum for LuLu), which would allow the text to be enlarged to about 138%, or about 15 points .
Other changes would require redoing the typesetting of the book. Currently I have just one typeset version for hardback, paperback and the e-book. Some of changes could be made to this with a large print version in mind, so the one original would work for all. As an example, a slightly larger font could be used for the standard editions, such as 11.5 point (up from 11 point) and slightly larger margins (1 inch up from .79). This would allow a larger large print version wile maintaining compatibility. Others require a different typesetting for the large print edition.
Times Roman is a serif font, which is not recommended for readability for those with limited vision. LuLu provide a limited range of fonts, which those in supplied PDF documents are converted to before printing (so it is best to start with one of these). Of the LuLu supported fonts, these are sans: Arial, Tahoma and Verdana. Of these Verdana looks the most suitable as it designed for readability at small sizes. Changing the font to Ariel adds about 10 pages to the book. Verdana is more generously spaced and ads about 20 pages. Increasing the font to 16 point would increase the book to 156 pages.
It might be worth changing the font for the e-book version as well as the large print edition. In fact it might be worth reversing the usual priority, where the print edition is seen as normal, and large print and e-book versions are derived from these. A standard size print edition with 11.5 point Verdana would look a little unusual, but be very readable.
Also adding 20 pages to the book might make it more marketable, with the customer feeling they are getting more (even if what they are getting is more white space). Originally I laid the book out to minimise white space, ignoring some printing conventions (such as starting a new chapter on an odd numbered page), so it looks a little crowded. Adding more white space, a larger font and larger margins would increase the book from 114 to 172 pages (a 50% increase).