Okay so I'm a bit of a fan - which after two books is quite an achievement. It's probably a little bit to do with the basic premise of genealogy being used to solve crimes (family tree research being an investigation in its own right after all), but mostly it's because Dan Waddell really can tell a bit of a tale.
There is a serial killer theme to BLOOD ATONEMENT, although initially it's only Detective Grant Foster who sees the parallels between the death of Katie Drake, and the disappearance of her 14 year old daughter, and a case 3 years earlier. But do not let that put you off, this isn't your standard mad, bad, feral serial killer. After Foster asks genealogist Nigel Barnes to look into the family history of these girls, the reason for this killer selecting his victims is finally resolved. It's the family tree research that gives Foster the leads he needs.
It's a real testament to the writing of Waddell that makes the idea of a consultant genealogist perfectly acceptable, it's the methodology in researching the past that makes for some of the real standouts of BLOOD ATONEMENT.
Whilst BLOOD ATONEMENT (and the earlier book THE BLOOD DETECTIVE) are effectively police procedurals, it's undoubtedly the spin of genealogy that creates the interest in these books. Having said that, at no stage does it seem like a trick or shtick or anything designed to make the books that little bit different. Instead the use of Nigel's skills seems seamless, makes sense.