Edward:
...
> The question is, can I read some *reliable* free ebook to enhance my
> knowledge of both Unix OSs and C?

Doing a quick search I find:
https://en.wikibooks.org/wiki/C_Programming
http://www.computer-books.us/c.php
http://www.cs.cf.ac.uk/Dave/C/CE.html
http://publications.gbdirect.co.uk/c_book/

Why not buy a used book:
http://www.zvab.com/servlet/SearchResults?an=stevens&sts=t&tn=Unix
https://www.ilab.org/search.php?headerAlltext=stevens+unix&search=Search

Other things that might or not help:
http://www.knosof.co.uk/cbook/cbook.html
http://c-faq.com/
http://www.lysator.liu.se/c/bwk-tutor.html
http://www.quut.com/c/c-www/
https://www.cons.org/cracauer/sigint.html
http://wiki.musl-libc.org/wiki/Alternative_libraries

Books I like:
Stevens / Advanced Programming in the UNIX Environment / Addison-Wesley
Harbinson Steele / C a reference manual / Prentice Hall
https://en.wikipedia.org/wiki/The_Unix_Programming_Environment
https://en.wikipedia.org/wiki/The_C_Programming_Language_%28book%29

Regards,
/Karl Hammar

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