Reddit mentions of Professional Baking

Not all cookies use the creaming method, but many do, so yes - a mixer is practically necessary.

Once you add the flour, over-mixing is absolutely a concern.

To really learn the difference, go study the Biscuit and the Muffin methods, and try some recipes with each. You'll be fascinated when you use the Biscuit method, as the 'dough' is practically dry flour and butter chunks, but it comes together beautifully. Muffins are similar, but you mix them just until wet.

Gluten development is what you're trying to avoid, this is also where using the right type of flour is important. Some call for Pastry Flours whereas others might use a Cake Flour, or even a Bread Flour.

Honestly, go buy a copy of Professional Baking and you'll learn 90% of what you need to know.

i'm the same - a hobby baker looking to get better. i bought this book recently

https://www.amazon.ca/gp/product/0471783498/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

and it's somewhat of a textbook-cookbook mix. has technical info as well as recipes. i'm looking forward to getting into it a little more, i've only flipped through it so far.