The C Book
An online version of The C Book, second edition by Mike Banahan, Declan Brady and Mark Doran, originally published by Addison Wesley in 1991. The version is made freely available.
While this book is no longer in print, its content is still considered very relevant today. The C language is still popular, particularly for open source software and embedded programming. Therefore, it is quite interesting, to people who use C.
The book is available also for download in PDF format, which is convenient for offline learning.