One of the platforms I was aware of before is called Flipsnack, where you can upload files that are written in Word, saved in PDF and uploaded.
Another option which I discovered more recently, is called OurBoox. It is an Israeli-based platform which claims to be the simplest platform for uploading and sharing books. You can read more about them in their "About" section. In order to make a book with OurBoox, you do not need to use PDF files. You can just have your students write their stories in Word or Googledocs, gather the photographs or graphics they want to use (be sure they have the rights to use the graphics and do not infringe upon others' copyrights) and then easily upload and publish them on OurBoox.
As a counselor for Digital Pedagogy for teaching languages, I have made a tutorial that shows just how easy it is to make a digital book. The tutorial is in Hebrew.
Have you ever had your students sharpen their writing skills by producing a digital book? What do you think of the idea? If you DO experiment with this idea, please fill out the feedback form so I know how it goes!
I hope you find this adds to your digital language teaching toolbox!
P.S. If you have any questions about the OurBoox platform , Mel Rosenberg is happy to provide support (in English or in Hebrew). Just drop him a line: firstname.lastname@example.org