I've heard it's light and chatty and gossipy from a friend, so have decided not to bother myself....
I thought it was a bit informal, but certainly not light, chatty or gossipy.
It was fast-paced and a bit chatty, but I didn't mind that. I wouldn't describe it as gossipy. I haven't read her book on Jane Austen, but Worsley definitely doesn't hide the fact that she's a Christie fan and is generally very sympathetic towards her. The writing is in a similar chatty tone to the one she presents her documentaries in, so if you find that annoying, you might not enjoy this book.
It's not the sort of in-depth study that you get with Laura Thompson. I would categorize it as being intended more for the reader who knows relatively little about Christie's life or someone who is interested but who might be bored by anything too exhaustive. I learned some interesting tidbits with regard to Britain of the time, but I can't say I learned very much. The newspaper reviews all noted her re-assessment of Christie's disappearance but again, I didn't find it particularly useful.