There are some tips to download your amazon kindle book to your computer, but it’s under azw3 format, it’s kindle format. Then if you do that, you need to kindle viewer to read. But it requires you to create a free account. So, if you don’t want to waste your time to any stuff like that. Just convert it to PDF or other eBooks formats.
for convert to other formats, you can use this site: http://www.zamzar.com/