Death Angel, Sweet Angel is the title of the "fictional" original story adaptation of Police Quest I (it sort of a blending of PQ1EGA and PQ1VGA with some references characters from later games: Ben Bulwer) in the Police Quest Casebook (both 1st and 2nd editions).
It is an original story based on the version that appears in the game itself, but takes some of its own direction and liberties with the characters and story (some events do not appear or are glossed over, and additional scenes added, characters names changed or merged with other characters). Some characters from later games are introduced into the events of the first.
It's worth noting that there are some changes between First and Second editions to the stories. Second addition is a bit more 'abridged' removing many scenes, and events (for example 1st edition has a run in with Helen Hots). Or certain details are changed for example second edition in part one "Roll Call" Sonny's opinion of Bulwer is a lot stronger than it is in the first edition.
Some of the changes was probably in order to make room for the new PQ4 content.
As there are enough changes between both versions of the story they can be found in below links.