While the original core books of Adventures Dark & Deep are out of print, the game can still be played. But how? You need a few books, and it all starts with OSRIC.
https://www.lulu.com/shop/stuart-marshall/osric-22-hc/hardcover/product-1yz9kqmm.htm
You can use AD&D, but the PoD copies on DTRPG contain errors, such as the number seven being transposed to one. It is much better to support the community, and the core game is a more accurate reflection using OSRIC. Plus, people are free to publish with OSRIC. It is not AD&D but close enough and open enough to do the same job or better.
Another option could be to use Swords & Wizardry with these books. You won't have the "first edition" experience, but it could be patched together and work fine.
https://www.drivethrurpg.com/en/publisher/3728/brw-games
The Book of Lost Lore has the Adventures Dark and Deep "extra content" pulled out and made the first edition compatible; this is all the extra races, classes, spells, and an optional combat system. This book was created for those who wanted the "extra stuff" of ADAD but not the base rules, so adding this to OSRIC will give you a good approximation of the original game.
You also get the skill rules here, which are vital to the game experience. Customizing your character by spending experience points for skills is a fantastic feature and takes OSRIC to the next level.
The second part is the monster book, the Book of Lost Beasts. This is a pretty easy recommendation and changes the variety of monsters in the classic game to add more unpredictability. You also get lower planes creature generation tables, so things can be even more random.
So you have a few extra classes, races, spells, and monsters? Is this the game? What is so different? Is it OSRIC-plus? Well, yes. We are getting close to, not the actual game, but this is close enough. Considering the actual game is not much different than "OSRIC-plus," it works well.
Where things start to go differently is adding in the third book, the Book of Lost Tables. This gathers most of the tables in a hex-crawl system, dungeon creators, town creators, wilderness encounters, and a bunch of tales in the ADAD Referee's Guide and puts them in one excellent book that is good for any game. This takes normal OSRIC and turns the game into an emergent gameplay system packed with charts to generate an entire world.
If you are not playing in a classic setting, procedurally generating one may be the way.
Want more? Swords of Cthulhu adds all the classic "removed from the DDG" Lovecraftian mythos and packs the game full of evil cults and terrible monsters. This is a fantastic book but also optional since some groups prefer to avoid mixing mythos like this.
Another book in the line of books is Swords of Wuxia, which has a "Mythic China" setting right, respectfully, and does not borrow the culture for some "made-up place." The setting is called "Mythic China," it isn't some borrowed culture like the "Egypt area" of Golarion or D&D's Kara-Tur. This is the real deal, which is done with careful research and respect for the culture. It is a "do not miss" book that immerses you in this fantasy world.
Finally, and most optionally, the Adventures Great and Glorious book adds rules for domain play, mass combat, and high-level play. It is an "end game" book that feels the most optional here but may be necessary once characters reach higher levels.
You can play an OSRIC-style version of Adventures Dark & Deep if the ideas are essential. The original game was a hypothetical extension of the first edition, so this gets you close enough with all the added bits of the out-of-print game.
You then have to ask yourself, are the ideas in ADAD essential? Do the extra races and classes make a huge difference? Some players may be hard-pressed to tell the difference between an OSRIC game and the ADAD version. Is it all first-edition, and that is what matters. The ADAD version of "OSRIC+" adds more of the unexpected and mixes things with new classes and spells. The old-school skill system is worth the price of admission. The book of charts is one of the best. The treatment of both Mythic China and the Lovecraftian mythos is top-notch.
As an OSRIC expansion, these books are pure gold; there is nothing like them in gaming.
