Up until this point, your "book" hasn't really been a book at all -- it has been a manuscript. The next step is to convert this manuscript into a printable form -- the baseline for what will actually become a physical book.
This is far easier today than it has ever been before. With a decent PC and the right desktop publishing software, you can create a printer-ready book in digital form. With this software, you can adjust margins and font size to get at your desired page count.
You can't simply send the printer a formatted Microsoft Word Document, because that type of word processing program isn't designed with the necessary print drivers to use a printing press. Instead, you need to format your book with one of two programs: Quark Xpress or Adobe Pagemaker. The software itself isn't cheap, and there's a steep learning curve, but a computer literate person can make a go at it with relative ease.
You also can't use one of the fonts that comes with your word processing program. For print-ready material, you need a Postscript font, coupled with Postscript print drivers.
With your desktop publishing program, you actually put together every single page of the book. Then you save all of this to a CD -- the desktop publishing file, the file for the font you used, and all the graphic files. This is your book, in digital form, ready for the printers.