First off, they badly combined the story of the second [find more alien tech] and third [Takashi ends up back on his homeworld] books. The first season roughly follows the first book with some changes in places for various reasons.

Second, because of how Sleeves work any character can change their body so Rick Flagg from Suicide Squad becomes Falcon from the MCU. This also means actors can play multiple characters when someone else uses the Sleeve which results in Mackie playing a sassy black lesbian at one point who is somehow more competent than who Takashi Kovacs is meant to be as a super soldier/spy Envoy.

Third, it's mostly a story about Kovacs love interest who really just wasn't anywhere near as prominent as the show makes her out to be, nor is she who the show makes her in regards to the Envoys.

Now while body changing does still fit within the lore of the story the choice of protagonist for the second season was shit because Mackie just isn't very good, regardless of anything else.

Ironically the best thing about the second season is Poe who isn't even meant to be in the story because it's originally Hendrix in the book but his estate didn't want him being used so changes were made, changes which were pretty much universally loved given the turnout of Poe.

Also the ending is dumb since it goes against one of the main points the Envoys stood for, but then because of how things turn out in the final battle someone probably wanted a happy ending to the show rather than how things were when the smoke cleared.