So I got this with high hopes considering how I liked Heavy's other supplements, but this really knocked off my boots. The upgrading system for items by the artisan NPC blacksmiths is very easy to implement and adjust for a DM allowing you to granularly apply various "levels" of blacksmith for players to hunt down or give a general "blacksmiths can do this" while allowing for players to really dig into customizing their weapons to fit play styles. With this that generations old longsword from Grandpa fighter Phil that the paladin's player has worked lovingly into their backstory won't get put by the wayside the second a +1 magic weapon drops.
In regards to the Runesmithing section, I don't think I've seen my player so giddy to engage with crafting downtime. This is expansive and allows for a lot of customization as well without being either too crunchy nor being too time consuming in game making players more likely to actually interact with it, looking at you other supplements where an enchantment may take 200+ days. That makes it useful for the players in game while also being a nice gold sink for the DMs to fiddle with. With the guild section also crafted into it, it also offers up a bit more roleplay opportunities, gold opportunites, and the like for players to engage with if they like as well beyond pure crafting rolls.
Overall this things get a huge thumbs up for fitting the feel of 5e in its simplicity and light bookkeeping while allowing for the ability for a DM to bring in customization options on their end.