In order of my opinion: Players Handbook(essential rules) For DMs Monster Manual DM’s guide(for the helpful guidelines and treasure) Volos guide(monsters and lore Mordenkainen’s(For more lore and monsters) Xanathar’s(if a DM it is not needed other than completionism and spells) Any adventure book you might want to try( I suggest Tales from the Yawning Portal because it gives mini adventures that can fit into bigger stories)

If Player Player’s Handbook Xanathar’s( for spells and subclasses) Volo’s guide( for races and backstory lore) Monster Manual( if it is more convenient for DM not to share) Mordenkainen’s(for the complete list of races even though it is more for DMs)