This book explores how we use language in the internet. it begins with a general introduction, describing lingusitics in general and sociolinguistics in particular. It discusses the internet in general and four particular aspects - email, chatgroups, virtual worlds (in multiplayer games) and the web.
The author clearly knows sociolinguistics very well and it is worthwhile reading the book just for that. However, his knowledge of the internet is weaker and often he relies on secondhand information that he is not able to evaluate well. This shows up in some of the more extensive quotes, repeated without much critical evaluation. For example, he frequently quotes 'Wired Style', only once (I think) refers to the Jargon File (without even working out who esr is) and doesn't mention RFC 1855.
There is not much original research here. For example, the chapter on email is based mainly on the author's own email correspondence, which is bound to be atypical. It would have been much more interesting to see if different communities, for example sampled from mailing lists, really used different varieties of language.
In some places it would be hard to distinguish between the language used in the internet and the language used by social scientists to describe the internet, between 'trolling' or 'boxen' and 'computer mediated communication' or 'cyberculture'. In others the author seemed unaware of features of the internet. For example, he does not seem to be aware that users can control how web pages appear or list emails by thread. Occasionally he gets terms wrong.
This is not a book to buy if you wish to learn the language (or rather languages) of the internet, but it is a very readable introduction to sociolinguistics applied to technology with some very plausible conclusions about where and how the internet will affect our use of language.