The setup for The Black Box is as follows: Detective Harry Bosch is working on a cold case that involves the 1992 murder of Anneke Jesperson, a Danish journalist. The murder took place during the May, 1992 LA riots.
The Black Box involves the standard elements of a Harry Bosch novel, including many twists and turns, and Harry's battles with the LAPD bureaucracy. If you've read Connelly's previous Harry Bosch novels, then you'll find The Black Box to be as tension-filled and engrossing as the past ones.
One thing I've noticed in this book is that Connelly is developing the character of Maddie, Bosch's teenage daughter. I think that the author has some long-term plans in mind here.
Harry Bosch, a Vietnam vet, is now in his mid-60s. He is currently working for the LAPD on a post-retirement contract; but he can only chase the bad guys for so many more years. At some point, Bosch has to retire and take up golf.
My guess is that Maddie will take over the Bosch family crimefighting practice just as Harry hangs up his badge for good. A young female detective won't appeal to all the old Bosch fans, of course; but this departure will also enable Connelly to attract some new readers.
Moreover, Connelly can keep Harry Bosch around in the background, as an "advisor" to his more vibrant, active daughter. This would potentially give Connelly the ability to retain a base of his older readers while drawing in millions of new ones.
I don't know what is Michael Connelly is thinking, needless to say; but I consider this to be a fair guess regarding the future direction of the Harry Bosch series.