First Steps
Wolfram Programming Lab (online)
Wolfram, ‘Elementary Introduction to the Wolfram Language’ (online, textbook)
Wolfram, ‘Fast Introduction for Programmers‘ (online)
Beginner and Intermediate Level Textbooks
Wolfram, ‘Elementary Introduction to the Wolfram Language’ (online) and related video tutorials.
Turkel, ‘Digital Research Methods with Mathematica’ & syllabus/course material (textbook, notebooks, exercises)
Wellin, ‘Essentials of Programming in Mathematica’ (textbook)
Wolfram Language Documentation (online)
Guillermo, Mathematica beyond Mathematics (textbook)
Subject Guides
Boardman, ‘Basic Image Processing in Mathematica’ (iBook)
Mangano, ‘Mathematica Cookbook’ (textbook)
Other Resources
Wolfram Virtual Workshops & Conferences (webinars, notebooks, videos)
Wolfram Community (online forum)
Mathematica Stack Exchange (online forum)
Wolfram YouTube channel (videos)
Wolfram Demonstrations (online, requires free CDF player)
Wolfram U (open courses)