I have learned some internal set theory (IST) from Lutz and Goze's Nonstandard analysis: a practical guide with applications. It is jam-packed with lots of interesting material, and has a nifty proof of the inverse function theorem. However, since it is a bunch of lecture notes, it is not as coherent as some other books, such as Robert's Nonstandard analysis or Nelson's own papers, his own unfinished book at http://www.math.princeton.edu/~nelson/books/1.pdf, or the probability book mentioned above.

So if you want to get excited about IST and get fun ideas for using it, read Lutz and Goze. To understand it, read Nelson or Robert.