http://publications.gbdirect.co.uk/c_book/ is reportedly a free book on C -- but it is C89, more or less.
If it is well written, we could look for people to update it to modern C. But if it isn't well written and clear, it probably isn't worth the trouble. If a few of you who are familiar with other good books on C would like to look through it enough to judge, and tell me your opinions, that would be useful.