There is an increasing interest in using C to program mice at the midlands micromouse and robotics club, and I was reminded about this Free C Book
As well as the web site, there are pdf's of the book which you might prefer to install on your ebook reader, tablet, laptop or phone.
It is quite old (circa 1991), and relatively traditional in its approach (but I learned my C in 1983 from K&R, so that's fine with me). It is based on the second edition of a published book by good authors who knew their stuff, so I've found it pretty good, and covers all the core language and libraries upto C90, which is what most folks use.
I hope people find it useful.