(Referring just now to michaelr's post)

Unfortunately, the book is out of print (the useful ones always are!) but can be found secondhand from time to time. It really is a very useful look at the way sessions have worked in the (recent) past, and it's a mistake, I think, to assume that there's necessarily a great deal of difference between the internal dynamics of a session based on geographical proximity as opposed to one based on a commonality of interest. The music will be different, but the attitudes won't.

For those who have assumed in contributing to this thread that there is something inherently "Irish" about the session as a phenomenon, I should perhaps make the point that what is often thought of as a typical session arrangement is relatively recent in that country. That's largely the result of geographical and demographic factors. It's England, where population tends to be more concentrated, and where social interactivity has more traditionally revolved around licenced premises, that has provided the model that we're most used to nowadays. Here in Sheffield, for example, we have (believe it or not) a tradition of playing dance music in local pubs that can easily be traced back to the 18th century; not continuous, of course, but with surprisingly few gaps. Same goes for the local Carol tradition.