In terms of pure ChickLit, try Erica James or - one of my favourites - Christina Jones
Have to second the Georgette Heyer suggestion; simply sublime. If you're into a bit of Regency combined with mystery, the Sebastian St Cyr series by CS Harris is much better than I thought it would be - I'm pretty fussy about historical accuracy. And if you want historical in different eras, then the Shardlake series by CJ Sansom (a history don at Oxford) is brilliant, and the Matthew Bartholemew series by Susanna Gregory is good too.
http://www.bbc.co.uk/news/uk-23691203
Should keep everyone going for a bit.