When Bosch was a young homicide detective, he worked with a partner named Frankie Sheehan, who always kept an old milk crate in the trunk of their unmarked car. He's carry it into every scene, find a good vantage point, and put the crate down. Then he's sit on it and just observe the scene, studying its nuances and trying to take the measure and motive of the violence that had occurred there.
-- Narration (chapter 3)

Importance: While this quote is discussing the habits of Bosch's old (now-deceased) partner, it is effectively a thesis statement for how Bosch himself conducts investigations. First, Bosch is incredibly diligent about collecting information at a crime scene via his fine-tuned observation of details. Also, Bosch is incredibly attuned to the relationships between mentor or partner detectives. Just as he once astutely learned the ropes from Sheehan, he expected Bella Lourdes to learn from him in a similar way...