https://wiki.mobileread.com/wiki/Sony_Portable_Reader
Check out the wiki. It has Content Formatting - Best Known Methods section.
If you need more help, include sufficient information. What format is the original book in? Without this information, its not really possible to give you a recommendation.
If the original is MS word compatible, then open it up in Word - set the default font size to 14-20pts and then "save as" an rtf file. If you right click on the document and open the properties dialog, you can set the Title and Author that the Reader will display in its directory.
If its a pdf file original, you have a much tougher job. No easy way to do this and keep all the formatting. It will change no matter what method you use because the Reader's screen is not the default letter page size that most pdfs are formatted to. Do you have a method for extracting text from a PDF? The free reader does not allow you to do this.