In addition to excellent series suggested above, I would like to add the following two.
- John Sandford - Prey Series
- Douglas Preston & Lincoln Child - Pendergast series starting from Relic
These are really good as well.
If she likes something light with some mystery elements thrown in,
- Charlaine Harris - Aurora Teagarden Series
- Lilian Jackson Braun - The Cat Who Series
are pretty enjoyable.