If you would like a book, I cannot recommend "The Art of Prolog" by Sterling and Shapiro enough. It gives a very understandable introduction to the language and is focused on teaching you how to approach different problem spaces while "thinking" in Prolog.
Regardless of XCode 4 free or paid, it would be nice to have a compiler separate from XCode. Installing the whole IDE you never use has always seemed like an overkill.
GNU Smalltalk supports both file based and image based development. Exporting source files is no means to an end — image based development is not widespread because it forces people out of emacs and vim.
Also known as MVC before Rails decided to redefine the term.