What's the reason for Chapters 7-12 not being available online?
So that you have to buy the book.
Keep in mind that the authors have invested a lot of time and effort into developing this material and would like to profit from their investment.
Making half the book available on line allows people to get a very good feel for what the level and scope of the book is, without having to request courtesy copies from the publisher, before deciding to adopt it for a course while still ensuring a reasonable revenue stream from those courses that do adopt it. Given the extremely reasonable cost of this text (about 1/5 of a typical engineering text these days), this is a refreshing and appreciated (at least by me) approach.