Player's Options include a point buy system. You literally can buy parts of your racial and class abilities, based on what you want within the points.
It also includes the precursors to the new approach to save with the addition of Sub-abilities, that include Will, Reflex, and Fortitude amongst them.
Each ability score has 2 sub-abilities, each corresponding to several new little things such as the parent ability does.
Also within them are rules for miniature based combat in Combat and Tactics, place new player races such as aarakocra, orc, orge, satyr, centaur, bullywug, wemic, etc.
In all there are 3 books, Combat and Tactics, Skills and Powers, and Spells and Magic.
There is also a DM's Option: High-level Campaigns, that fits in with those somewhere.
Unless you want to add all those things you may not want the full 2.5, but you can pull some interesting stuff from them.
If you can find it, and use computers, you may want to see about AD&D CoreRules CD-ROM 2.0, as it has 19 books including options for standard and Player's Option character creation, plus comes with Windows Help file as well as HTML and RTF version of all the core books, plus MANY of the brown Complete series splatbooks.
Whichever is cheaper is what I would go with for books or CD-ROM, but I warn you the expansion to the software can run near $200 if you can even find the 2.0 version to begin with.
AVOID 1.0 AD&D Core Rules CD-ROM like the plague!
If you are familiar with it, then 2.0 to 2.5 is sometimes as big a leap as 3.0 to 3.5, maybe the same as 2.0 to 3.0 as it is VERY closely touching on things that became 3rd edition.
Also 2nd edition contained a revised version of the books, the black books, that just cleaned up some things, but it is still 2.0, and the 2.5 people refer to also have those black covers.
Here are the books for 2.5 AD&D
Advanced Dungeons & Dragons Archive: Player's Option: Skills & Powers
Advanced Dungeons & Dragons Archive: Player's Option: Spells & Magic
Advanced Dungeons & Dragons Archive: Dungeon Master Option: High-Level Campaigns
Advanced Dungeons & Dragons Archive: Player's Option: Combat & Tactics
That is also the importance on the order I would give to get them. First being most important to play 2.5, last being least important, but you may decide otherwise.
Myself, I just pick bits from them without using 2.5 as a system, just like any other book outside of the PHB, DMG, and MM.
