Best D&D/RPG materials that I missed after a long gap?

[2D8HP]

Recently I have been buying all the Wotc & some third party 5e D&D books I can find after a very long hiatus from D&D and RPG's in general. I started RPG'ing around 1978 with the Bluebook/Basic D&D box that had the "In Search of the Unknown" module included, (while I can't remember if the PHB was out, the Monster Manual was already out, but the DMG wasn't). My circle drifted to other RPG's in the 1980's and by the early 1990's it seemed that the only RPG's anyone in my area would play were "Cyberpunk" and "Vampire", which just weren't to my taste, mostly ending the hobby for me.
Early on I had all the D&D/AD&D materials I could get my hands on (including "Chainmail" and "Empire of the Petal Throne") but 1985's "Unearthed Arcana" I did not get (just seemed like too many cumbersome changes), and since I already had OD&D and the earlier "basic" set I did not get the 1981 and later basic/BECMI materials, and I completely skipped 2e.
I did get alot of 3e books, some 3.5, but no 4e material (in fact while I still get most of the earlier fantasy fiction and Monty Python references in this forum the Anime and computer game stuff largely goes over my head and reminds me of my age, so yeah if you live long enough time travel is real)).
Of non D&D RPG's the ones that most interested me (but didn't get to play) were:
1. Pendragon,
2. Castle Falkenstein,
3. Space 1889, and
4. GURPS "Swashbucklers".
I have little patience with reading non character creation rules (combat rules especially bore me), I read RPG books mostly for setting information.
My tastes seem to run more towards mundane (at first) characters in Fantasy world's rather then super heroes or supernatural characters in modern settings.
Since there is about 20 years of RPG material I largely missed, what should I go back and get?

[JAL_1138]

I'd suggest looking at 2e.

2e had some really great settings (Planescape, Spelljammer*, Dark Sun, Dragonlance**, Ravenloft***, a massive expansion of Forgotten Realms including such sub-settings as Maztica and Al-Quadim, etc.) and really cleaned up 1e's byzantine combat and surprise rules while retaining cross-compatibility with very little work (to convert, give 1e monsters a THAC0 based on their HD and relevant bonuses or penalties, and do the same for PC-class NPCs based on class, level, bonuses and penalties, and you're pretty much done. Or use the to-hit tables from 1e; it works out ok if having two DMGs on hand doesn't bug you). Later books like the Complete [Whatever's] Handbook or Complete Book of [Whatever] lines were really hit-or-miss but greatly expanded character-customization options.

*Spelljammer is crazy-awesome, emphasis on the crazy. Silliness (including of the basic premise) abounds.
**Dragonlance is a good setting...with unfortunately highly-railroady published adventures. Skip the modules, get the setting books, if you want to get into it.
***Ravenloft technically made its first appearance as a one-off module in 1e, but was expanded to a campaign setting in 2e.