OpenGL Books on the net

Can anyone tell me why books like “The Red Book” or the blue one, are free to the net for everyone to read. There is no license protecting them or they are free from the author. Because if it is illegal to read one of this unless you paid it, i would like to know where i can find other books related to opengl and computer graphics programming without paying for them.
Let’s say for example “OpenGL Game Programming”.

Thank you for your explanation.