The Amazon Trivia interactive menu on the left side of the screen says "The seventh season drew from the Bosch books The Concrete Blonde and The Burning Room." I didn't know there was a 7th season yet, but the Harry Bosch Wiki says " It is likely to air in Summer 2021 dependent on the impact of the Coronavirus pandemic on filming." https://harrybosch.fandom.com/wiki/Season_7 This trailer has good information about which books are sources for each Bosch season.