So KDP is kindle direct publishing, you can publish your own book, just upload the interior and the cover, write a description up and sold on amazon. I'm just selling notebooks and planners. Easy to set up and no up costs.

Same process for higher content books like your own. To get sales make sure you do some good keyword research and maybe advertising

Lego is yeh buying when discounted wait until retired and reselling it on amazon

My earnings on here were a bit of a pittance to be honest