To filter your results so you see only e-books, choose "e-Books" under Filter My Results.
When you find a book that interests you, click on the Online Access link. If access to the e-book is limited to one or a few simultaneous users, this information will be listed:

While you can certainly still assign a reading from this book to your students, they may have to wait their turn to access it - just like if it were a physical copy on reserve in the library. But an e-book that doesn't mention limited access will not have this problem. (Interested in seeing if it's possible to increase the number of simultaneous users for a book you've found? Contact your subject librarian!)
Another place you can start your search is our Books & E-Books Directory, which is browsable as an A-Z list of our major e-book packages, or you can limit the list to specific disciplines.
To link to an e-book in Canvas or other site or email, you will need a "permalink" — a URL that is not tied to a specific search session. Click here for our guide to Finding and Creating Permalinks.
There are many e-books in the public domain, too - check out the Internet Archive and Project Gutenberg for millions of texts.