murder mystery mono monday!

Shameful to say but I've never read any Agatha Christie before! But we've currently on a mission to find copies in charity shops/library sales as my 10 year old niece has decided she wants to read them!  She's currently working her way through the Harry Potter series - think she's read most of them in just a month! - and loved reading the Murder Most Unladylike series so now wants more murder mysteries in her life! Any recommendations for the best ones to read gratefully accepted!

So this is the first we found in a library sale for 25p today - maybe I'll have a nose through it first! ;)