Serious scholarly and technical books have indices and serious readers use the index heavily. They do so partly to avoid reading the books more than is necessary. Because we read books efficiently by not reading them more than we have to (you can probably tell at this point that I am deeply under the influence of the very wise, short and much talked about but little read book: How to Talk about Books You Haven't Read).
Although scholars will often browse and read a technical or deep book by filleting it and raiding it through its index, the index also serves the obvious and primary purpose of enabling us to search books that we have already read, and locate specific pages. The index of a traditional book was there because we did not have search engines. We did not have searchable texts. However, indexes are still a very attractive way of entering and browsing a serious book. For example consider the index to Lawrence Lessig's Code Version 2.0. As soon as I glanced at that book's index I wanted to see what Lessig had to say about Berners-Lee.
If you click on that fragment of the index, you will jump straight through to the index where the page references are live (this becomes clear once you have clicked through to the index page, where all the page references have bounding boxes, this does not show up in the clipping which is just a fragment of a JPEG, but a fragment that links); since the book is Open Access you can immediately navigate from the index to the page where Lessig notes how the inventors' ambitions for the web grew when they realised that with an open system documents could link to anything, to any document or any object in the HTTP network
What lesson can we draw from this example? One lesson is that links are the core of the web. The original point of the web is to facilitate citation and linking. But the second lesson that I draw from the example, is that indexes in printed books are still a very good way of approaching the books especially when they are digital documents. Authors and scholars working in the traditional mode were building links to their text when they made their indices. They were working in the way of the web before there was any possibility of a an instant live linking of documents, one with another. It could well be that one of the most valuable aspects of a digital book in the Exact Editions format is that the index is fully linked and the navigation of a book via its index is even easier in digital format than it is in printed volume form. It is obvious that the traditional Table of Contents is a crucial way to navigate a digital book, and so the pages in the ToC should be live-linked, but it may not be so obvious that the index is equally important and helpful to deep readers. Sure, we know that we can search the whole book very easily, but a good index helps us to interpret a book through the author's perception. Very valuable.