I honestly have found the free "An Elementary Introduction to the Wolfram Language" to be the most helpful resource for learning the basics, and if you download the Notebook edition version, you can open each of the chapters in Mathematica and work with them straight from there. https://www.wolfram.com/language/elementary-introduction/2nd-ed/
I also have this one and have found it pretty useful: Mathematica®: A Problem-Centered Approach https://smile.amazon.com/dp/3319275844/ref=cm_sw_em_r_mt_dp_VJC58JBR1VX9HX4CABKR