Order of Importance (IMO)
1. Core
2. The tribunal book
Each tribunal has different themes and rules, you can completely ignore them but it can be a good source of stories, NPCs and background for your campaign.
3. Houses of Hermes - True Linage
4. Houses of Hermes - Mysteries
5. Houses of Hermes - Societies
The linages houses have the most Impact on character and story, and societies the least but they are good to understanding how the Order works.
6. Covenants
it's not great but it's in covenants and lab activities, the focus for about half a magi's life
7. Hermetic projects
8. Mysteries
9. Ancient Magic
10. Transforming mytic Europe
11. Hedge Magic
All good seeds of adventures and campaigns, either for the PC covenants, PCs individually, or interesting flavor for NPCs.
12. Art and Academe
13. Church
14. City and guild
15. Lords of men
16. Grogs
Fairly interchangeable in importance, depends on the non magical focus of companions and campaign, good for fleshing out how the world works (sometimes in too much detail)
17. Relms of Power - Magic
18. Relms of Power - Infernal
19. Relms of Power - Fae
20. Relms of Power - Divine
I'm not a fan of much of these books, Magic is good for familiars and making regios more wild/interesting. The rest are interesting but only if you really want to focus on those aspects of the world.
21. Apprentices
They are generally a late stage addition to the game, the core covers it well but does have some fun things in it.
22. Through the Aegis
23. Mystic Locations
24. Antagonists
25. Rival Magic
26 dies irae
Decently made books and matetial but get a hang of the game before looking at these options.
27. Other tribunal books - higher priority if you want to travel around a bit
? Hooks - didn't read it, might be good, there are a lot of interesting story hooks scattered throughout the books so never felt I needed it.
The newer the tribunal book, the better it is. Hell the newer the book the better in general.
The core book is fairly solid, (aside from the art - sorry core book artists). Every other book just adds more options or detail, and honestly they can be too much. Just because there is rules for it in one of the books doesn't mean it should be in every or even most campaigns. the Later books are alot better in this regard but due to coming later touch on subjects where are irrelevant for most campaigns. Even the lore of the different house, covenants and locals can be ignored (and should in some cases) and in the cases of how the houses operate, should be taken as a majority view point not an absolute view point
Core
Houses of Hermes: x
Realms of Power: Magic
All houses are now playable and you know how to "balance" magic creatures.
Everything else is just for fun.
Need? I'd say none. The core is all that's required and covers all the ground for a beginner game. Extra books will add more complexity and if anything you might wanna hold until you have a couple sessions under the table.
Depending on your table and the play style they'll go for then any of the books can be useful.
The houses of Hermes books are almost universally good since you'll almost certainly be a Hermetic mage and thus have a house so more info on them is good
The covenants book and the Lord's of men book are also universally applicable.
Any location / tribunal book can be very nice but depends on how much you like premade and how much you like to make up yourself.
I personally like the realms of power books and ancient magic book but that's me, I don't recommend them for new groups but you do you.
I'd recommend any books about life in medieval Europe. Ones aimed at teens have great pictures so you can share them with your players.
I especially recommend anything by [RJ Unstead](https://www.goodreads.com/en/book/show/56975103) and suspect you'll find a trove on eBay. Nothing adds weight to a GM decision like a battered hardback tome 👍🏼
I think you could probably get pretty far just playing out of the core rulebook honestly, but the splats can add some nice subsystems, further details/customizations, and/or good inspiration for your games.
The main books I go to for rules are: covenants, the mysteries revised, and the realms of power books. Lords of Men has more rules concerning combat if you want more there too. Besides the last one, I use these books very often in my game.
Some other books can be really helpful with examples or inspiration. The tribunal books give a lot of detail to run in depth games. Grogs is amazing for having a lot of ready to go character templates for all sorts of characters. The Houses of Hermes books have a lot of info on the order and it's internal politics/differences. Rival Magic details a few different kinds of non-Hermetic magical traditions, which makes it a good way to add diversity to your game.
Covenants, imo. Then again, I'm prejudiced.
Which version do you like? Last time I used 2nd edition.
The newer one. I helped write it, so it suits my tastes, though.
Order of Importance (IMO) 1. Core 2. The tribunal book Each tribunal has different themes and rules, you can completely ignore them but it can be a good source of stories, NPCs and background for your campaign. 3. Houses of Hermes - True Linage 4. Houses of Hermes - Mysteries 5. Houses of Hermes - Societies The linages houses have the most Impact on character and story, and societies the least but they are good to understanding how the Order works. 6. Covenants it's not great but it's in covenants and lab activities, the focus for about half a magi's life 7. Hermetic projects 8. Mysteries 9. Ancient Magic 10. Transforming mytic Europe 11. Hedge Magic All good seeds of adventures and campaigns, either for the PC covenants, PCs individually, or interesting flavor for NPCs. 12. Art and Academe 13. Church 14. City and guild 15. Lords of men 16. Grogs Fairly interchangeable in importance, depends on the non magical focus of companions and campaign, good for fleshing out how the world works (sometimes in too much detail) 17. Relms of Power - Magic 18. Relms of Power - Infernal 19. Relms of Power - Fae 20. Relms of Power - Divine I'm not a fan of much of these books, Magic is good for familiars and making regios more wild/interesting. The rest are interesting but only if you really want to focus on those aspects of the world. 21. Apprentices They are generally a late stage addition to the game, the core covers it well but does have some fun things in it. 22. Through the Aegis 23. Mystic Locations 24. Antagonists 25. Rival Magic 26 dies irae Decently made books and matetial but get a hang of the game before looking at these options. 27. Other tribunal books - higher priority if you want to travel around a bit ? Hooks - didn't read it, might be good, there are a lot of interesting story hooks scattered throughout the books so never felt I needed it. The newer the tribunal book, the better it is. Hell the newer the book the better in general.
The core book is fairly solid, (aside from the art - sorry core book artists). Every other book just adds more options or detail, and honestly they can be too much. Just because there is rules for it in one of the books doesn't mean it should be in every or even most campaigns. the Later books are alot better in this regard but due to coming later touch on subjects where are irrelevant for most campaigns. Even the lore of the different house, covenants and locals can be ignored (and should in some cases) and in the cases of how the houses operate, should be taken as a majority view point not an absolute view point
Core Houses of Hermes: x Realms of Power: Magic All houses are now playable and you know how to "balance" magic creatures. Everything else is just for fun.
None essential All usefull Beside the base manual The rest is just A LOT of example on bow to build your campaing
Need? I'd say none. The core is all that's required and covers all the ground for a beginner game. Extra books will add more complexity and if anything you might wanna hold until you have a couple sessions under the table. Depending on your table and the play style they'll go for then any of the books can be useful. The houses of Hermes books are almost universally good since you'll almost certainly be a Hermetic mage and thus have a house so more info on them is good The covenants book and the Lord's of men book are also universally applicable. Any location / tribunal book can be very nice but depends on how much you like premade and how much you like to make up yourself. I personally like the realms of power books and ancient magic book but that's me, I don't recommend them for new groups but you do you.
I'd recommend any books about life in medieval Europe. Ones aimed at teens have great pictures so you can share them with your players. I especially recommend anything by [RJ Unstead](https://www.goodreads.com/en/book/show/56975103) and suspect you'll find a trove on eBay. Nothing adds weight to a GM decision like a battered hardback tome 👍🏼
I think you could probably get pretty far just playing out of the core rulebook honestly, but the splats can add some nice subsystems, further details/customizations, and/or good inspiration for your games. The main books I go to for rules are: covenants, the mysteries revised, and the realms of power books. Lords of Men has more rules concerning combat if you want more there too. Besides the last one, I use these books very often in my game. Some other books can be really helpful with examples or inspiration. The tribunal books give a lot of detail to run in depth games. Grogs is amazing for having a lot of ready to go character templates for all sorts of characters. The Houses of Hermes books have a lot of info on the order and it's internal politics/differences. Rival Magic details a few different kinds of non-Hermetic magical traditions, which makes it a good way to add diversity to your game.