It provides a fairly clear explanation of five different private language arguments that philosophers have supposed Wittgenstein offers in PI 258. I intended it to function as a good introduction to the whole private language argument topic.
If I ever get round to writing a book on the Philosophical Investigations, it will form part of that book.
Incidentally, the clearest intro to the Investigations is still Marie's McGinn's (though I don't agree with McGinn on e.g. the private language argument - see my paper). I have just ordered Block's How to Read Wittgenstein - will let you know what I think.