Frankly, Google is your friend. Enter the gobbledigook word and add “3D printing” and it should give you links to explanations of the terms.
YouTube is also your friend. There are hours of videos on various aspects of 3D printing that you can learn from. Frankly, I don’t think you need a book (not easily searchable) and online courses, while available, will basically only cover what you can get from YouTube for free.
Are you actually starting with PETG? I’ve never used it, but it’s my understanding that PLA is more beginner friendly.
Take things slowly. Don’t go too nuts with Thingiverse. Not everything on there works well. Some of it is flawed or requires more finicky settings/supports/etc.
Start with small(ish) things. Big things bring their own problems.
For heavens sake, don’t go nuts with “upgrades” until you are very familiar with how your machine operates under normal conditions. Also, you need time to break a machine in. Beds will warp. Build plates will become less “sticky”. Wheels will collect dust and debris. You need to learn what “normal” problems look like before you go adding “upgrades” and introduce a host of new problems.
Tell us what printer you have. There are a number of useful (sometimes even “necessary”) upgrades that you can print for yourself that improve a printer but don’t affect it’s performance. These can be a good way to add useful things to the printer and give you some experience.
Before you add one of these “upgrades”, ask people who have the same printer what version of said upgrade they printed. That will tell you A) that the upgrade you have in mind actually works as expected, and B) that the file from thingiverse actually prints reliably on your printer.
As an example, I’m thinking of a model (can’t remember specifically what it was) that I found on Thingiverse. I downloaded it, put it in the slicer (Cura) and discovered the original was larger than my print bed. No problem, I though: just scale it down to size. Well, it turns out this particular model had a built-in hinge. When you scale it down, you also scale down the gaps in the hinge and, after nearly a day of printing, I ended up snapping the hinge off trying to get it to flex. The plastic in the two parts of the hinge were now so close together they fused. This is one of the things I mean by not going nuts with Thingiverse: some things are really meant for specific printers or require extra skill on your part to get it to work on yours.
Problems like this can make learning a frustrating experience. So, as I say, start small, ask others if they’ve printed the model and better yet, if they’ve printed it on your model printer, and feel free to come back here with any problems / photos / questions.