Don't bother, it's just a sample chapter.
For anyone interested, here's an Amazon link to the full book (surprise, it's not free): https://www.amazon.co.uk/Mastering-Linux-Advanced-Penetration-Testing-ebook/dp/B06ZXT1DPG/
Honestly one of the best Linux programming books I can recommend that covers lots of aspects is http://www.amazon.com/GNU-Linux-Application-Programming-Series/dp/1584505680
It's good for beginners, but points you in the right direction to dig deeper when required.