When Bosch and Maddie go to the gun range Maddie says that she wants to shoot a zombie target. Maddie, played by Madison Lintz, also played Sophia on The Walking Dead where she turned into a walker (a zombie).
Two characters, Julia Brasher (Annie Wersching) and Eleanor Wish (Sarah Clarke), are current and historical partners respectively, of Harry Bosch (Titus Welliver). Both of these have also played partners of Jack Bauer (Kiefer Sutherland) in 24 as Nina Myers (Sarah Clarke - seasons 1 - 3) and Renee Walker (Annie Wersching - seasons 7 & 8).
In a scene where Bosch takes his daughter to a shooting range, the target paper represents a zombie image and Bosch has a line of dialogue about how she "popped that zombie right between the eyes". This is clearly an in-joke as Bosch's daughter is played by Madison Lintz, who had a role on the zombie-themed series The Walking Dead (2010).
There are three actors who have also acted in the series 24: Annie Wersching, Sarah Clarke and Steven Culp.