Well-ploughed fields of wisdom

In other words, a book that has been used more or less on a daily basis for over 20 years.

This morning the radio was playing 'I don't like Mondays' (how very predictable, as would have been 'Manic Monday'). Poor Mondays, I thought. So if I had any musical talent, I'd probably write a song about how Sundays are the worst since they're the ones that usher in Mondays after all.