Is the version or edition you recommend most? Sounds quite fun!
Pendragon 5.2 is pretty much the definitive edition, since Greg Stafford has sadly passed away. There is a 6th edition in the works based on his notes that has potential, but the last time I checked, it wasn't nearly ready. There's a bunch of books for 5e and almost all of them add a lot to the game.
Apart from the main rulebook, there is:
Pendragon Grand Campaign: The actual heart of the game. It's a campaign that takes players from the era of Uther Pendragon uniting Britain and fighting Saxons to King Arthur's last battle at Camlann. It will take several years worth of weekly sessions to finish, but it's absolutely amazing and totally worth reading on its own.
Book of Uther: Expands the start of the Grand Campaign by 10 more years. Not really necessary, but it can make certain events in the Grand Campaign easier on the players.
Book of Knights and Ladies: Almost mandatory, a lot of expanded chargen options for knights from other cultures.
Book of Battles: There's a lot of army battles in the Grand Campaign and this gives them a lot of extra depth.
Book of Manor: The game assumes the default player knight is landed and this book gives options on how to customize their home manor and also expands manor economy beyond the harvest being decided by GM fiat and the current conditions as set in the campaign.
Book of Estate: An expansion for the larger landowners. Apart from expanded rules for family members aging and dying, it's not necessary.
Book of the Warlord: Another expansion for playing even larger landowners. Again, unless the players are into that, it's not needed.
Book of Armies: Really useful for the GM, since it has stats for a lot of enemies that can be encountered in battles.
Book of Entourage: A smaller book covering expanded rules for squires and all other types of hirelings you can take with you on adventures.
The only absolute must from the extra books is the Grand Campaign. Uther, Battles and Armies are very useful. Everything else comes with extra bookkeeping, so it's really up to any potential player group if they want to deal with that. Pendragon has a lot of bookkeeping: a player will probably want to have one or two alternate knights in case something happens to their main one. Even if you get invested in a family line, it's absolutely possible for a knight to die with their heir being five years old. Plus you have to roll for every adult family member surviving the year once they hit certain age. And your wife has a chance of dying in childbirth every year and small children are also prone to dying. If you want to play a female knight, it's better to focus on the Chastity virtue.