I believe that this site is legal,
http://www.vb-tech.co.za/ebooks.htm
you can download text versions of various children's novels, including some by Enid Byton. To get a better format, use Word, create a template the same size as the prs 500 screen, save as e-reader template in properties, then do a "save as" template.
Drop the text (copy and paste) into the template; edit the font size (it looks much larger on computer than it is on ereader, try pt18), justify margins, and "save as" name of book RTF.
I hope this is clear. If I could work out how to do it, I'd upload the RTF template I created for the prs 505, which is roughly the same size as the 500.