INDEX (optional, not recommended)
When I buy a "real book" that is not fiction, one of the first things I do is look in the back to see if it has an index. To me, an index is critical.
However, it would be a painstaking, time-consuming and worthless task to do this with an ebook - because built-in search capability makes an index un-needed. With an ebook, such as the one you're reading, you can use the built-in search capability of your ebook compiler software to make it a snap to find any word you want.