Some freely available books, some out of print, some still available in online stores, others in plain HTML.
Thinking in C++ 2nd Edition (2 Volumes) by Bruce Eckel - This is probably the most famous one. There is also a printed version on sale.
An Introduction to GCC, by Richard M. Stallman - A GNU C and C++ manual for those who want ot get started with the GNU compilers, gcc and g++.
Nectarine City Handbook of C Programming Style by Joseph Miklojcik – A book on writing clear code.
The C Book by Mike Banahanm Declan Brady and Mark Doran – The online version of “The C Book”, published by Addison Wesley in 1991 (no longer in print)
Introduction to C Programming by Rob Miles – A online introductory C book.
C Elements of Style by Steve Oualline – Building good programming style
C++ Annotations by Frank B. Broken – Moving from C to C++