The staple books were the skills & powers and spells and magic. C&T added combat options, but weapon mastery pretty much catered to the fighter.
The character building approach gave you points for race, class, and proficiencies. Any points you didn't use early on you could use later. This was pretty much the first attempts at REALLY making humans a worthwhile race, since humans usually ended up with more points to put in their class.
Races and classes had a list of abilities and points that you could use to buy them. In some ways this worked better than GURPS or its ilk, because you couldn't spend your points backwards. Thus to gain fighter class abilities, you pretty much had to give up fighter class abilities.
Where it fell down IMO was that the classes were not themselves balanced, and some classes had big wads of points that were intended to be used on one thing, but you could spend on other, more productive things.
Cleric was the biggest offender on this score. Cleric had like 125 points. The intent was that you would spend a big chunk of your points buying clerical spheres. In reality, it was a lot more economical to only buy a few speheres, and then grab the abilities that gave you fighter abilities and wizard spells instead, making you an uber munchkin.
IMC, I limited the abilities you could buy according to your deity and required you to spend most of your points on spheres, and it worked like a charm. I did various other tune-ups and it worked great for a long time.
I really felt like PO was "the system" to play D&D under for a long time one you blocked off the loopholes; the fact that S&P separated abilities into bins made it far better than GURPS and its ilk IME.
But two players who were determined to abuse the system convinced me that to run point based systems will always require GM intervention, even when you improve the situation with "bins" like PO had. Which is probably why I am such a big fan of classes to this day.
Still, I could see 3e/d20 using some elements of PO. Not building it from the ground up as PO did; I am absolutely convinced that is prone to abuse. Rather, I think you could take half a step in that direction, assigning a point value to all the class abilities, and then allowing the player to swap out limited numbers of the class abilities for optional abilities. That, IMO, would work.
Spells & Magic also had a lot of variant magic systems and was probably the best magic book for 2e.
DMO: High level campaigns had lots of good advice for keeping campaigns challenging, and the scion/paragon "templates" were intrumental in keeping my campaign running to that end.