No, this isn't an advert, it's a AB-er recommendation - there's a difference, you're not going to benefit from the post yourself (unless, of course, you ARE David Crystal!)
I've got his big fat English Language book, which is very good. But he seems sometimes too crotchety to me - picking fights with other authors who don't agree with him. Keep your squabbles to the teachers' common room, chaps.
I love his 'The Stories of English' and 'The English Language,a guided tour.'
I hope you enjoyed his talk, Mark.
Does he have a really 'cultured' voice and look like a don?