Most are pretty good, although some are better than others. The short guides cover the basics quickly, while the general guides tend to have references to resources available on the net as well as explanations of the various Internet applications. Companies or individuals wanting to do business and commerce on the net can get some advice from these books, but the quality of many is not good. Even without getting into technical details, there are specialized reference topics on the Internet that merit their own books. These books stick to a certain theme, but aren't just techie texts.
Email is only one Internet application, but probably the most important.
Unfortunately, none of these books are really up to the task. It's really only a small part of the net, but there are a ton of books written about the World Wide Web and browsers, HTML, Java etc. Since the Internet got popular, lots of people have wanted to give their opinion on the Information Superhighway and other cliches . Most of these are uninformed, but some have an idea or two of value. Some books just don't want to fit into a topic.