Ok I'm going to back-up some of these opinions.
First off, Ruins of Symbaroum is amazing. The classes are light weight and most of your identity will come from the feats you pick, of which these classes get more of. That mirrors Symbaroum's original classless design. The classes are balanced, simple, low magic, and interesting. They also convey ideas of the setting very well, which IMO is why I actually prefer the 5E rules to the original Symbaroum rules. Furthermore, a lot of stuff in here is usable outside of Symbaroum; corruption rules, magic items, feats, monsters, and so on are all ready to be planted into any setting, fully compatible with normal 5E. Not the classes, but everything else. Also, their bespoke bestiary book is way better in RoS then in original Symbaroum; the original Symbaroum monsters are communicated through feat and come off as boring. The RoS versions are much more interestingly designed and require little overhead to run.
Next, we have the Deadmen's Guide of Dragongrin, by Absolute Tabletop. This is my favorite dark fantasy but not too dark setting book in 3PP atm. It is called by Abtab an "Altas of Inspiration;" it is a setting guide where you get some fluff but mainly you get a huge amount of roll tables to customize each region to your own liking. This includes generating antagonists, locations, history, big events, direction, and more. The book is huge, with lots of moddable monsters in the back, as well as a really sick premise. Essentially, a former hero learned of a great disaster in the future. He became evil to stop that disaster, and has become a Chronolich who manipulates time and the future in order to forever stave off that disaster. This hero named himself the Dismembered Lord, defeated all the heroes in the setting, and now has his own tyrannical empire that he controls via time-manipulation, all dedicated to preventing an extinction that he himself may very well be. Compelling stuff, and the roll tables can make anyone's game richer for using them.
There's a lot more I could talk about. I've read now probably over 200+ books for 5E from the 3PP sphere in the last decade. There's so much good, so I'll start going through my stash and collecting more names of stuff.