As a long time 2020 player, who has all of the source books (or at least the pdfs), Night City is the only book beside the core rule book that is essential. Night City may be the best setting made for any RPG. It details the city for GMs and gives numerous hooks or ideas for short adventures to long term campaigns. It is clear that more time and effort went into this sourcebook than any other produced for the 2020 line. While I would argue that most of the 2020 sourcebooks are valuable, this one is a must. Get it and enjoy.
[5 of 5 Stars!]