Rules References
The fifth edition of D&D has three official rulebooks, each of which was first published in 2014:
Player's Handbook (2014) (abbreviated PHB)
Monster Manual (2014) (abbreviated MM)
Dungeon Master's Guide (2014) (abbreviated DMG)
The free [https://www.dndbeyond.com/sources/dnd/basic-rules-2014](Basic Rules (2014)) contains portions of those three books.
Official Rulings
Official rulings on how to interpret rules are made here in the [https://www.dndbeyond.com/sources/dnd/sac/sage-advice-compendium](Sage Advice Compendium). The public statements of the D&D team, or anyone else at [https://dnd.wizards.com](Wizards of the Coast), are not official rulings; they are advice. The tweets of Jeremy Crawford (https://twitter.com/jeremyecrawford), the game's principal rules designer, are sometimes a preview of rulings that will appear here.
A Dungeon Master adjudicates the game and determines whether to use an official ruling in play. The DM always has the final say on rules questions.