I always find good tutorials on the subject, but it almost always is C++. Is it hard or just doesn't make sense to make GUI applications with C?
A good book, guide or a tutorial is fine.
Platform: Linux
Use GTK+. It's written in C and exposes a well-designed C API. Their tutorial is quite excellent.
GTK+ is the toolkit for GNOME, so your applications will fit right in with many Linux desktops.