Articles on the Catalan

https://en.wikipedia.org/wiki/Catalan_Opening

https://www.chess.com/article/view/chess-openings-catalan-opening

https://www.chess.com/article/view/the-open-catalan-system

Here are the most up-to-date books on the opening:

The older series by GM Avrukh has the Catalan in Volume 1

The series called Wojo's Weapons is based on the Catalan which is also in Volume 1 of that series.

GM Nigel Davies's book might be a good place to start as it concentrates on move orders and limited exposure to the lines. 

Similar in spirit to the Davies book above but more detailed is Keep it Simple 1.d4

This book has been transformed into a Chessable Course with the same name and author. 

Chess.com Database on the Catalan

https://www.chess.com/openings/Catalan-Opening

Modern Chess Site

https://www.modern-chess.com/en/chess-databases/database=31

Chess Publishing Forum Link

https://www.chesspub.com/cgi-bin/chess/YaBB.pl?board=catal

Chess365 Database

https://www.365chess.com/eco/E00_Catalan_opening

YouTube Videos on the Catalan

Dereque Kelly Introduction to the Catalan

https://www.youtube.com/watch?v=pQI-DWNe_2I

Jim's Chess Channel- Open Catalan

https://www.youtube.com/watch?v=okgoorPZLXg

Jim's Chess Channel - Closed Catalan

https://www.youtube.com/watch?v=HiKifJo8pwU

Chess.com Introduction to the Catalan

https://www.youtube.com/watch?v=A0GBJn5Ro54

The ChessWorld

https://www.youtube.com/watch?v=eI7aIto_4yQ

thanks, @CornerPawn

Attack with the Catalan! - Chess.com