Heyer's First Mystery May 8, 2009
Heyer was still quite young when she wrote this. For experienced Heyer readers it will seem a bit immature, but she does a good job nonetheless. (Immature Heyer is better than 95% of anyone else's better work.)
It has some of the standard features you find in pre-WWI British mysteries (e.g., old, complex manor, complicated terrain, secret passages, physical danger, chase scene, and ,of course, romance). However, the characters are well drawn and multifaceted.
A good read.