Actually, I have the big four, books that is, from AD&D. I have the Dungeon Master's Guide, The Player's Handbook, and the Monster Manual 1. I also have Monster Manual 2 but that's of lesser importance for me. More of an accessory than completing the necessary set really.
The DMG is the primary book of the game. The Player's Handbook is second. The Monster Manual 1 is an obvious third and while some people have suggested they really only think that the first two are actually necessary, I would disagree a bit.
On the outset, it seems that all the monsters from the DMG are what is in the MM1, thus making MM1 a bit redundant. However, the MM1 includes information that is actually necessary for using those characters. Containing details that are not included in the DMG that help making decisions regarding those monsters.
Depending on the type of GM you are will let you know how necessary the MM2 really is or not. If you are like me and 90 percent of your games are home-brewed, then the MM2 is more of just an accessory for extra monsters.
If you play a lot of published modules though, there are frequently monsters from the MM2 that are included in them that makes the MM2 now a more important book to have.
In my estimation, Fiend Folio isn't used very often in published modules thus relegating it to un-necessary. The same goes for Deities & Demi-Gods. They are both interesting and can be of use to the home-brew GM, but otherwise, eh.
Unearthed Arcana, in my one person's opinion, is the least useful of the bunch and doesn't include anything that improves the game. It does include a lot that can dramatically change the game if you include them. For me, changing the game but not really improving it, or worse, making the game less fun, well, there's just no point in even having that book.
This is where OSRIC really shows it's true value and it's stated mission. The above books are propriety, commercial products that you have to buy. Fine if you have the money, bummer if you don't.
However, with the existence of community web sites, blogs and forums, home-brewed monsters, modules and ideas can be shared among folks freely. Especially if presenting them in the context of OSRIC.
Because OSRIC is designed to be an open gaming version of something like AD&D, more can be legally shared publicly among gamers allowing the game to grow again. Because of that, I am changing my own WikiMage website to make my published characters, settings, etc.. OSRIC compliant, thus able to legally share and invite everyone in.
Now, granted, I may be the only one to think the stuff on WikiMage is worth using, but you never can tell. Even a stopped clock shows the right time twice a day, it's possible that something I have posted of my own creation could be useful to another Game Manager ("DM" technically is a proprietary reference so I am told).
Anyway, as I make Wikimage more OSRIC compliant, think about stopping by, feel free to use what you find there. I would also encourage people to join me as an editor of Wikimage so they can add their own OSRIC compliant material and make it an even better site.