What should I do if I can't transfer my books to my Kindle?

Try connecting your Kindle via USB cable to your computer. Another thing to try is to change the format that the e-book is in. The most widely recognized format of e-books is EPUB, which is a format that Kindle does not recognize. This was due to an update a few years ago to force people to only get e-books from the Kindle store for their Kindle devices. You can get conversion software online for free called "Calibre E-book Management". This software allows you to convert EPUB files into MOBI (files recognized by Kindle devices). You can also use the software to change things about your e-books, such as the font size and cover photo.