The editing and publishing of diaries is a rather specialised field. They’re not quite straight history, as no detailed references and sources are required, and they’re not quite memoir, as they’re in the moment, not looking back with the benefit of hindsight. And there tends to be a lot of dross to weed out.
So what makes for a good diary for publication? Adrian Moles and Charles Pooters of this world aside, diaries of nobodies are rarely of interest. And yet those truly at the centre of power are unfailingly dull. The best are by those who are well connected. They must write well, be brilliant and engaging but maybe somewhat thwarted and unfulfilled. Nothing sharpens the pen like ego without sufficient outlet.
Some diarists genuinely write for their own amusement or as an exercise. Michael Palin is a good example. He’s kept diaries since his early 20s and it’s an automatic reflex. For Palin, writing a diary is pilates of the pen. The motives of others are maybe less pure, and I would argue a good diarist needs an excess of what Graham Greene referred to as that "splinter of ice" in their soul. The other thing about good diarists – they’re generally entertaining; the type of person you want to sit next to at dinner, but more than a little untrustworthy or unreliable, even if you count them among your dearest friends.