Sophie Kinsella's I've Got Your Number, which was simply delightful.
I've read all of her Shopaholic series, and also her standalone books published under the pen name Kinsella. She also has several books published originally in the UK only under her real name (Madeleine Wickham), and I haven't read any of those. That puts my page count at around 200 (I was already half-way finished before the readathon started).
I've Got Your Number was predictable in a very Sophie Kinsella kind of way, but it kept my attention nonetheless. It was actually a wonderfully humorous romance, which is what she's great at, if you're in the mood for a fast, fun read. Her female characters are always slightly ditzy (but actually quite smart), her males are always wonderful or terrible (in a very Bridget Jones sort of way), and all the English-speak makes her novels delightfully amusing. A full review will come later, of course, but suffice it to say I wholly enjoyed I've Got Your Number -- and it was the perfect easy, light book to begin a readathon.