D&D Books

The Player's Handbook and the Dungeon Master's Guide are the most important books for a DM to have.

I would personally add the Monster Manual to the most important books list but I also am not familiar with how much of its contents is available for free. For me those monsters are more universally useful than what you find in Monsters of the Multiverse. 

Quote from Allurian219 >>

I would personally add the Monster Manual to the most important books list but I also am not familiar with how much of its contents is available for free. For me those monsters are more universally useful than what you find in Monsters of the Multiverse. 

The Monster Manual is important too, though a surprising amount of stuff from it appears to be available on D&D Beyond as part of the Basic Rules (see here for a filtered search). It's definitely worth picking up a physical copy of it if you like physical books, but if you prefer digital resources you may be able to get by without it, especially if you're new to the game.

The Basic Rules can get you very far without needing to purchase any books, but the only ones I feel are essential are the three core rulebooks. I think the 2024 revision is great (LOVE the new Monster Manual) , but having the original 2014 core books are fine too. The new 5.5e Dungeon Masters Guide does come with the Greyhawk setting and better maps, so you wouldn't need another setting book like Forgotten Realms, Ravenloft, or Eberron unless you like the flavor. I am really enjoying the new Arcana Unleashed and I think it compliments the 5.5e Player's Handbook very well, but that is completely optional.

Quote from CaptainJinxed >>

As a DM with practically zero D&D books, what kind of books are most important? I have Mordekainen's Monsters of the Multiverse on here and the Player's Handbook 5e, but I got that before I joined Beyond, so it doesn't register on here. What books should I buy?

 If you're planning to run on DDB, somebody in your group should have the digital PHB for the version you're running, and somebody else should have a master-level membership. These don't have to be you, but it usually ends up being you. (You don't absolutely need them, but not having PHB access really limits player options.)

For a new DM, the new DMG seems to be pretty good, but you can live without it.

The monster manual is nice for having access to more critter stat blocks, but you don't need it, especially since you have MotM.

If you're running a premade adventure/campaign, that's nice to have digitally.

Everything else is optional.

Agree with all that is above - PHB, DMG, and MM are all good purchases, with so

Moving beyond the core three, I would highly recommend the two Griffon Saddlebag Books.  While they are third party content, the creator clearly understands and respects game design.  The items are generally much more interesting and unique than official content produces, but tend to be relatively commensurate in power for their associated rarity.  Great way to get a lot of content you can give your players to spice up your game.

Unless you really like homebrewing monsters book that contains a significant amount of monsters, whether first or third party, also is extremely helpful to a DM.  More monsters means more options for encounters and less time you have to spend making creatures unique on their own. For monsters, I tend to prefer third party content - the monsters are more dynamic and powerful, leading to more challenging and interesting encounters.  Frankly, I think Wizards’ big failing with 5e has been incredibly flat and uninspired monster design, which the 2024 MM entirely failed to fix.

Flee Mortals! Is my go to recommendation for a quick monster book.  It both has fun monster content and reintroduces and reimagines the 4e concept of Minion to allow you to run significant adds that remain threatening even at high levels.

I would stay away from third party player-facing content.  A LOT of it is unbalanced and/or poorly designed, with significant power jumps or (worse in my opinion) subclasses that do not stay in the lane of their class and trample on the toes of other players.  Until you are a confident enough DM to recognize power and differentiation issues, and have the ability to rebalance to reduce problems and feel bad moments, I would avoid this content.

Lastly, if a book interests and inspires you, whether that is the setting information, the monsters, the art, whatever, that is a good purchase.  An inspired and excited DM runs better games.

I’d also ask your players what characters they want to play (classes, subclasses, species, backgrounds, etc.) and look up whatever information or purchase whatever digital books you need if you’re using DDB you need.  The 2024 PHB has a hefty selection of options so start there.

Personally, I'd not buy too much into "poorly designed/balanced" talk.

Buy what you want, &, solve in-game problems w/out-of-game talk.

Premature bans & reactionary red feedback is how we get boring bare-bones Banneret instead of Purple Dragon-riding Purple Dragon Knights.