Read the Mallory series for the depth of the characters. The latest book is The Chalk Girl, where homicide detectives, Mallory and Riker hunt down a killer leaving bodies strung up in Central Park. Coco, a small child found abandoned in the park, is unable to tell them where she came from or who she belongs to other than knowing a great deal about the rats streaming through Central Park on what was turning out to be a very unusual day. Read in order starting with Mallory's Oracle.
See the order here on the Stop You're Killing Me website: http://www.stopyourekillingme.com/O_Authors/O'Connell_Carol.html