The information contains in the book makes it a pretty good primer on SIP. The organization of the book and its content is very good. The language and ways in which the author explains certain things are confusing. I think the author should be able to do a much better job in explaining things. I often find myself having to re-read certain explainations twice and much carefully when I feel I shouldn't have to -- the topics are not complicated but not well explained. Someone like Doug Comer or Richard Stevens would probably be able to make it a much easier read.