I just got through watching all 6 seasons of Bosch on Amazon Prime. As someone who read all of the Connelly books on which the series is based, I found that (1) the TV show was MUCH better than what I was expecting, and (2) they completely changed plot elements from the books so as to keep those who were readers of the books guessing. Many of the details were the same as in the books, many were changed, the goal of the writers was to keep you guessing whether you read the books or not, and for the most part they did succeed in keeping me guessing.
They totally changed the Jerry Edgar character from the Connelly books. In the books, Edgar is a total "by the book" cop for the 4 or 5 books in which he is Bosch's partner. He grows weary of Bosch's continual bending of the rules and, eventually, leaves the police force and becomes a probation officer. In later books, however, Bosch and Edgar become friendly again when he needs Edgar's help on a case. In the TV series, Edgar starts out as the "by the book" cop, but eventually Bosch's influence leads him to cross some lines that the "by the book" cops do not cross. The same trust issues that percolate between Bosch and Edgar in the books are also observed in the series, although by season 6 it's clear that the partnership is on very solid ground and most of the trust issues have been overcome.
The "Money" Chandler character is actually an amalgam of 2 minor characters in the books- one is Honey Chandler, the Police Commissioner, and the other is the Mickey Haller character's ex wife Maggie, a prosecutor who is nicknamed "Money" because she wins all her cases. One of the other things they change from the books is that in the book Angels Flight, Honey Chandler is actually the person who is having an affair with slain civil rights lawyer Howard Elias. Bosch discovers the illicit extra marital affair between the police commissioner and the attorney who makes a living suing the police, privately confronts Chandler with it, and uses this information to extort Chandler into revealing privileged and confidential information to him to assist the Elias murder investigation. None of that happens in the TV series. In the TV series, Elias is having an illicit extramarital affair, but it is with a random, young, hot, white intern he hires, and his wife knows about it and apparently doesn't have huge problems with it. This is totally different from the book.
Also different from the books is the Irvin Irving character. Although he is a political animal in both the books and TV series and recognizes Bosch's talent in both the books and the TV series, he is actually presented as a much more likeable and humane character in the TV series, and this may be due in part to the outstanding performance by Lance Reddick playing that part. Reddick played a similar cop character (Lt. Daniels) in The Wire, who is not as politically savvy as Irving. In the books, Irving and Bosch butt heads much more ferociously than in the TV series, where Irving is shown to become increasingly personally beholden to Bosch.
I thought Titus Welliver was an interesting choice to play Bosch. I saw him in Sons of Anarchy as the Irish gunrunner and he is not the guy I would have pictured casting in the Bosch role. But I think he ends up capturing the character as Connelly wrote him. And Jamie Hector (who was Marlo Stanfield in The Wire) is fantastic as J Edgar. Really, really good casting there and with Reddick as Irving.
Finally there is a big difference between the books and TV series on the "Crate and Barrel" characters. In the books, "Crate and Barrel" are presented as goofball, comical detectives whom Bosch only trusts to interview old ladies and rope off crime scenes. He does not trust them to do anything that requires any conscious effort at thinking. In the TV series, "Crate and Barrel" provide comic relief to the other characters, especially Barrel, but they are both shown to be competitive detectives and Bosch trusts them to do much more than he does in the books. Crate and Barrel play key roles in solving some crimes in the TV series.
Overall, the series is excellent and can't wait til season 7 is released February 13.