Why can't my friends view the HTML e-books sent by me?

If you want your friends to read all this content, you should send all these files or compress the entire output folder to him /her, instead of sending the index.html file in the folder. Please don't modify their relative path, or others can't read it normally.

PS: Maybe you should pay attention to the relative paths among the HTML and other multi-media files. Try to send all the output files to your friend to make sure he /she can browse them smoothly. Or you can upload them to your own website, then all your friends can browse them online without sending them.