I don't find weird 'edgy' stuff much use, and I'm not currently in the market for Venger-style sleaze or Pundit's historicism; fairly meat & potatoes stuff with a typical OD&D/Greyhawk vibe rather than B/X - so more demons & snake cults, fewer BX standard monsters like giants, probably preferable. The heavy OSR emphasis on BX type material leaves a bit of a gap for the former.
- Bill Silvey's Book of Lairs 1 & 2 review: https://knights-n-knaves.com/phpbb3/viewtopic.php?f=26&t=10791
- Guy Fullerton's Q about what to call "lair raid"/traditional dungeon crawling: https://knights-n-knaves.com/phpbb3/viewtopic.php?f=3&t=9336
- Core rulebooks: the holy trinity: PHB, DMG, MM; OSRIC rule book; etc.; these could be books or boxed sets, but define the DM's minimum investment required to play the game
- Supplemental rule books---these build out the rules further in terms of expanding upon the original footprint of the rules with new types of rules/content like in D&DG, MotP, DSG/WSG, et al, as well as expanding the amount of material within the original footprint of the rules as the setting books like UA, OA, GA, FRA, etc. do, and as new monster books do in FF and MM2, Monsters of Myth, Malevolent & Benign 1 & 2, Dwellers in Dark Places, etc., which also obviously fall into this category, as do magazines since they generally focus on supplemental rules (or adventures); books like WotC's Primal Order series, Bard Games' Compleat Alchemist/etc. all fall into this category, as does Trent Foster's AD&D Companion
- Adventure modules: modular modules, either stand-alone or in a series (which tend to be bigger and get complied or published as super modules: GDQ1-7, A1-4, Guy Fullerton's F1-4, Jeff Talansian's ASSH modules, etc.); many modules often introduce new supplemental rules material as well with new monsters, magic items, spells, classes (although less frequently), etc.; WG6's new to hit tables and attrition rules fall into this category too (since they're not really extensive enough to warrant calling their inclusion in the module as a supplement too, in my mind)
- Adventure supplements: modules that are also supplements; S4 Lost Caverns of Tsojcanth is the best example of this content type since its Book 2 was a mini-MM2/UA rolled in with the main 32 page module, but others definitely exist too, including ones where the new rules are required to play the adventure (which is not the case with S4): Q1 (with its myriad of rules for planar travel in the Abyss), B1-2 (with their guidance on how to run and play adventures, although one could argue that this content belongs in the core rule books), and S3 (with it's Gamma World-like technology usage charts); more-recently, Anthony Huso's Zjelwyin Fall fits this category with its Astral rules and codifications
- Setting Books: these book obviously detail setting-specific content a la the World of Greyhawk, Blackmoor, Tekumel, et al, which can include sourcebook material about gods, spells, races, classes, etc., in addition to the usual geography, history, languages, runes, clothing, culture, and such
- Content Tools: these tools aid a DM (or perhaps a player but I mostly think of these are DM/referee aids), so this is a much more narrow label in my head than most folks probably think of accessories, and includes books like Monster & Treasure Assortments, Dungeon Geomorphs, and Rogues Gallery, as well as books of tables like Dungeon Dozen, Tome of Adventure Design, the d20 era Ultimate Toolbox, much of Kellri's CDD netbooks, with Gabor's recent Nocturnal Table being a recent example. Midkemia Press' Cities also lands into this category, although most of their other city books would be adventures or supplements. On the player tools front, the only example that really jumps out to me is for CoC, with the S. Peterson's Guides to Creatures and Creatures of the Dreamlands books; but while written as field guides for players to use for their characters to ID Mythos horrors, in practice I always used the books as referee to show players a picture of the creature that their characters were confronting....
- Accessories: character sheets/adventure logs, DM Log/campaign mgmt tools, JG's Ready Reference Sheets, DM Screen (low content unless there's an adventure or set of reference sheets with it), as well as graph/hex paper, miniatures, and dice---although Inkwell's dungeonmorphs dice and Flying Buffalo's Adventure Dice certainly provide some content; I suppose books of maps without keys/adventure content for them would fall more into this category vs. content tools too
- Guidance Books: this one's a bit fishy to me, but I think that there's a category for books about how to play in general, how to play in a certain style/setting/tone/edition, best practices in adventure design, etc.; my still-unpublished dungeon design essays book will fall into this category, as does EGG's Role-Playing Mastery and Master of the Game books, and perhaps his Gygaxian Worlds series; this category would not include gaming history books like Playing at the World or Designers & Dungeons or general summary/catalog books like Heroic World or JEH's Fantasy Role Playing Games, since those don't usually impact how you play the game at the table; magazines sometimes focus on this kind of content in their articles, too
My sense is that the OSR largely focuses on core rules these days, with supplemental rules and adventures being the next categories that see the most content focus (although if you look at the number of dice, cards, and miniatures Kickstarters it's entirely possible that Accessories are the tail that wags the OSR dog, perhaps?). At the Knights & Knaves Alehouse I think we lament the lack of focus on good adventures (which is a tone/quality issue rather than a volume issue), and also see a lack of content tools as a critical gap that drives down the supportability of AD&D/OSRIC as a platform.
Some additional discussion on Knights & Knaves Alehouse at https://knights-n-knaves.com/phpbb3/viewtopic.php?f=3&t=16157, and on reddit at https://www.reddit.com/r/osr/comments/dcdlu1/what_types_of_products_does_the_osr_lack/, and on Twitter at https://twitter.com/AllanGrohe/status/1183766019090845696