It roughly adapts the stories. They're all in different orders though and so they play out a bit differently and with the consistent cast of characters that sometimes means there are new people involved to shade the cases in slightly different ways than the book versions. Additionally some storylines in the show are presented as backstory in the books, but since they're present day here it also creates a different dynamic. Each season typically pulls from 2-3+ different books, but there are also some more minor plot threads sewn in that are wholly original, typically relating to the other characters apart from Bosch.
Interesting. I was a huge of Connelly back in the '90s and up the mid '00s (I didn't really stop being a fan either, I just haven't kept up with the novels) so Bosch has always been something I should check out. Welliver is perfect casting.