In his own words,
"In contrast to the ubiquitous presence of C programs and systems, good knowledge of and about C is much more scarce."
His book is a wonderfull example of up-to-date knowledge available in the public domain, it is licensed licensed a Creative Commons “Attribution-NonCommercial-NoDerivatives 4.0 International” license. The content of the book is divided into levels, and this is one of the characteriscs of the work which makes it suitable for both beginners and professionals.
The last version of the book is from Feb 13, 2018. Available to download: http://icube-icps.unistra.fr/index.php/File:ModernC.pdf
Other than the book, the author writes a blog about programming, https://gustedt.wordpress.com/, which I do recommend as well.