If you’re starting learning (or refreshing yourself) C programming language then I recommend 2 excellent modern books.
- C for Dummies, 2nd Ed, Dan Gookin.
- Modern C, Jens Gustedt.
The C for Dummies is easier to follow if you’re beginner in programming in general. But the Modern C gives more explanation and some of the inner concepts of how and why.
Both books are available as hardcopy and/or digital ebooks. If you buy a physical copy of ‘Modern C’ then you can get the ebook version for free.