This is a nice, light, gently political romance novel written entirely in emails and letters. I would recommend it as an undemanding holiday read - as long as you are the sort of person who can suspend disbelief easily and will be able to buy the hero's emails as something a middle-aged male MP might conceivably write to a middle-aged male friend in the ministry (I just couldn't).
I have a soft-spot for epistolary novels and have read a few recently (and indeed had a go myself at writing one) but it's a difficult form to master. You have to be able to ventriloquize convincingly or the whole concept falls apart. The Moonstone has some wonderful letters and other documents in it and The Guernsey Literary and Potato Peel Pie Society, which I read not too long ago, does a pretty good job of it and is worth a look.
The problem for an author working in this form is not just finding characters that fit the story but finding ones who fit and might conceivably write a letter (and indeed say in it what needs to be said in order to get the tale told).
For example, can you imagine an MP - or indeed anyone, male or female - writing this to a friend, particularly one who is a senior colleague?
"How I wished I had been an unreconstructed Old Labour chauvinist so that I could have taken this blissful opportunity to do what I was longing to do - sink my hand into those dark ringlets and pull her head against my shoulder, and then bury my nose and mouth in the warm scent of her hair."
"She'd got her hair scooped back from her face this time, in some kind of rather fetching arragement, but wayward tendrils kept escaping at the sides. It is just one shade away from black, and her eyes, I decided after some very serious analysis, are not exactly hazel, more grey, but with little spangles of gold. And her skin is simply amazing - so white that it's nearly translucent, with a delta of tiny blue veins just visible near the corners of her eyes."
It's clearly written by a woman (specifically one who is trying to write a romance novel). Occasionally, embarrassingly, Thornton flags up that these emails are 'by a man, you know' by making the MP comment on the heroine's "most breathtaking breasts." Ick.
Also how many men do you know who a) could reference Good Wives by Louisa May Alcott appropriately b) would feel comfortable making such a reference to another man and c) would expect that other man to 'get' the reference anyway.
My favourite epistolary novel is Fraulein Schmidt and Mr Anstruther by Elizabeth von Arnim, which does away with this problem by only including Fraulein Schmidt's letters. It works really well.
So, read that, then, rather than More than Love Letters. Or, failing that, Lolita.