Will be DMing and have been for a few sessons.which books do you consider essential

I brought most of the books on DnDbeyond before realising I have to literally buy them again on here. I would assume PHB, Monster Manual, Can and maybe either SCAG or Xan

Hi Valentine, SCAG is not yet available here. Other than that, I'd say your instincts are good.

Well, keep in mind that the only thing buying books here does it make it so that you don't have to input things manually.  It doesn't change anything as far as how the game plays.  I do strongly recommend the monster manual so that a large amount of making monsters is done for you already.  it changes 20 minutes of work into a single search, drag, drop function that takes but a few seconds.  it is the single biggest quality of life improvement for anyone designing their own adventures.  If you're purchasing modules, not so much, since they all come with all of the monsters / npc's to run them.  Personally, I play with three tabs open at all times, the game, the settings page for the game (specifically the api page), and dndbeyond for quick reference.

The PHB is essential, and a great asset in Roll20. Personally, I would get a campaign before the Monster Manual. A campaign will give you a huge range of assets, including maps, tons of varied monsters and magic items. It'll have NPCs with backstories in Faerun that you can repurpose all over the place, even if you don't intend to run the campaign. The art assets are all useful. Maybe just as important, a purchased campaign will give you a good feel for how to organize your GM space in Roll20.

In my opinion, out of the official WotC Books and Modules I would suggest getting them in this order - Monster Manual, Players Handbook, Volo's/Xanathers/Mordenkainen books (any order for those 3 depending on which one you think you will get more use out of) - Also Kobold Press has Tomb of Beasts and Creature Codex which are incredible resources for additional critters. After that a good adventure from one of the publishers, or tile sets and map packs from indi creators. If you are running one of the planned adventures, like ToA or SKT you can push that forward in importance as it provides everything you will need to play that game. If you are running your own campaigns, then get the core books with compendium features and look into getting tile sets and art assets to add to your games.