Table of contents for an ebook

I am working on Ebook and want to display the table of contents of the eBook. It will have units, chapters in the unit, topics in each chapter.

Any best practices to display the Table of contents for an easy feel to the students? Ideally the Table of contents will be on each page on the left hand side. I am thinking of having an Expand option prefixing each unit, chapter and topic. If user would like to expand, then it can be expanded. Else it will show short content i.e; for only units. If anyone has good ideas it really helps.