Good point, however, there are no free textbooks that are made by collaborative effort. This book is also free to edit, so teachers can take parts out, add parts and so on.
This book is definitely written "for people to read". It is very clear and understandable. One could think that a text book written over one weekend is very messy and hard to read, but this was indeed not the case. The writers especially focused on making the book as easy to understand as possible.