Well, my suggestion for the Ultimate Spellbook, would be as follows:
- Spells organized by Source with Subcategories: Core+Spell Compendium(Unquestionably allowed every single game ever) ; Supplements(Completes, Champions of X, ToB, Magic of Incarnum, etc.); Setting Specific (Faerun, Eberron, Planescape, Ravenloft, Kingdoms of Kalamar, Dragonlance); Third Party (SubSubcategory: Dungeon & Dragon Magazine) ; Online Articles ; Third Party websites. - Not sure if humanly possible, but include a category with all spells available from free sources? SRD and Planewalker.com plus the Online Articles come to mind. This would be most useful for starting/poor game groups.
- List with spells per Subschool... That would be pretty awesome.
- Index searchable by descriptor? [Evil] [Mind-affecting] [Cold] [Vile] [Taint] etc... Mighty handy for specialized mages.
- In an Index, would it be possible to define each Class/PrC as Arcane casting or Divine casting or both, and classify the spells as Arcane only or Divine only? Mega useful for classes that can access all divine or all arcane.
- List of class-specific/domain specific spells? Sorcerer/Wizard ONLY (not present on any other list in any source), Sorcerer ONLY, Wizard ONLY etc... etc...
- Bold the lowest level at which a spell is available. Example: Haste Trapsmith 1 Sorcerer/Wizard 3. - Mighty handy for scrolls, wands, magic items and the like.
- If a searchable index is ever made... I request a 30ft tall statue made of solid gold of each one that contributed to the making of the ultimate spell book.