Destiny Discover - great for fiction and nonfiction eBooks. You can also see physical books in the library on this app. 

View a tutorial on checking out eBooks on Destiny Discover.

Sora by Overdrive - great for fiction eBooks. It has a wide variety of of popular books, and lists and recommendations to help you discover new reads. 

View a tutorial on checking out eBooks on Sora.

Mackin Via ebooks - good collection of eBooks (especially nonfiction) that can be checked out by many students at a time.