I think it's a common enough occurrence that when we re-read something we find new stuff in a text. We either notice stuff we missed the first time around or we can connect it to new thoughts in our head. That's why I reread the 1st edition DMG from cover-to-cover every couple of years (Though it's been more like three right now. That's grad school for you, I guess.). And there's a line from an Frantz Fanon piece that I didn't understand when I was assigned it as an undergrad and I didn't understand it when I was assigned it again the first year of my Masters degree. Just last spring I was assigned the same chapter a third time and I think I've finally got the gist of that one sentence now, but I don't understand it well enough to trot it out casually in a discussion, or to make it a key component of a paper. Maybe that'll come later, after further wrestling with the piece some more.
Which brings me to an idea I've been pondering for a while now: levels of spell comprehension. Because the world needs one more way to make M-U's more complicated, right?
Level 1 Comprehension
At this level you don't understand the fundamentals of the spell enough to memorize it. You can roll to cast it (see below) only if you have your spellbook open in front of you, at the right page. If you blow the roll you cannot attempt again without further study (i.e. try again tomorrow, chump).
Level 2 Comprehension
You can memorize the spell, sorta, but you have control problems while casting it and must roll with every casting. If you fail the roll the spell is gone with no effect.
Level 3 Comprehension
Call this Basic Mastery if you like. You can memorize and cast the spell, just like in normal D&D play.
Level 4 Comprehension
As per level 3, but you may be able to retain the pattern sufficiently for multiple castings. Every casting after the first requires a roll to cast. Once you fail you are done for the day. Kinda like DCC.
Level 5 Comprehension
As per level 4, but you do not lose the spell if you blow the casting roll. Wizards in the original Chainmail work like this, I think. Those dudes just fling fireballs all over the damn place.
Level 6 Comprehension
Ultimate Mastery. You no longer need to memorize the spell to cast it, but you must roll when it isn't memorized.
SOME ISSUES TO CONSIDER
ISSUE #1: What to roll to cast?
There are lots of ways you can go with this. An Int check. An Int check with a penalty equal to the spell level. A Spellcraft check for you weirdoes that use non-thief skills in your game. You can adopt the Chainmail 2d6 system, which would look something like this:
2-5 Fail - Snake-eyes sends you to some sort of mishap chart, maybe? 6-7 Delay - Spell goes off on your initiative next round. 8-12 Success - Boxcars results in the spell being overpowered and out of control, perhaps.
-1 for a first level spell, -2 for a second level spell, etc.
+1 caster level 1-2 +2 caster level 3-6 +3 caster level 7-8 +4 caster level 9-10 +5 caster level 11+ If you want caster Intelligence to figure into the 2d6 roll, here's a chart for that:
Int 3: -2 Int 4-8: -1 Int 9-12: no modifier Int 13-17: +1 Int 18: +2
Another alternative would be a percentile roll. Dave Hargrave's PHUMBLE PHACTOR math gives a 50% chance of success, +2% per level over 1st, +5% per point of Dex over 12 and -5% per point of Dex below 9. The other percentage system you could use is Empire of the Petal Throne:
Given the comprehension rules above, this system is actually quite generous to high level casters, so -5% or -10% per spell level would be in order here.
Note that EPT has an extra stat that modifies your spell casting percentages, Psychic Ability. Here's my 3d6 version of it (the rest of them are here). You could sub in Intelligence for Psychic Ability here if you didn't want to introduce a new ability score.
PSYCHIC ABILITY 3-7 Non-Psychic Cannot use spells or magic 8-9 Barely Psychic Cannot use spells greater than 2nd level 10-11 Average Psychic Cannot use spells greater than 4th level 12-13 Somewhat Psychic +5% spell casting chance 14-15 Quite Psychic +10% spell casting chance 16-18 Highly Psychic +15% spell casting chance
(Hargrave and Barker both list percentages to fail to cast, but I re-expressed them as success chances because that makes more sense to me.)
ISSUE #2: Any old spellbook = Level 1?
As a DM, you need to ask yourself if you want the MUs in the party to be able to open any old grimoire and start casting random shit out of that bad boy. I say thee nay, since I generally think spellbooks not as handy users guides, but as the highly idiosyncratic journals of madmen. They take some time and effort to sift through, and maybe read magic and/or write spells depending on the rules systems. And I'm pretty sure I read once many years ago a serious occultist (Al Crowley, maybe?) who claimed that the Goetia only works properly if you hand copy out the entire text before your first summoning. The effort of making your own manuscript copy is what gives the text its occult juice.
On the other hand, maybe you want to trick the party into summoning Orcus's Mom. That's easier to pull off if MU's can skim any old musty tome and get Level 1 comprehension.
ISSUE #3: Starting spells
This one is easy for me, since I run '81 Basic. BX D&D allows for newly minted MU's and elves to possess a single pitiful level 1 spell in their book. I can do all sorts of things and it will totally be a step up from the rules as written. Example: all arcane PCs get one spell at level 3 mastery [per the standard rules] plus two at level 2 and three at Level 1.
IIRC my Labyrinth Lord rulebook allows starting MU types to possess a second level spell (or maybe two) in their starting spellbook. Under the LL rules they can't do a darned thing with that magic until level 3. Set the comprehension of those spells at L1 and the new PCs can begin using them right away, IF they dare to bring their spellbooks to the dungeon AND page through it in the middle of a combat.
For AD&D1, which gives players 4-5 first level spells at the start (and lots of cantrips, if Unearthed Arcana is your bag) you might have the player simply roll d4 for each spell. The same thing would work for straight up OD&D, where I usually assume that starting PCs begin with all first level spells. If you are feeling brave, change that d4 to a d6!
The free spells you get when you level up would come at L3 comrpehension, while spells added to your book might call for a d3 roll.
ISSUE #4: Advancement
How does comprehension increase over time? Two main options spring to mind: allocate some points or roll some dice. Each time you level up you get so many points (say, your new level plus your Int mod, minimum 1) and each comprehension advance costs points. Maybe double the current level of comprehension plus the spell level.
Or maybe upon leveling you roll a d for each spell in your book. If the result is higher than the current level of comprehension, advance it to the die roll. Once per level-up you can trade in two d4 rolls on spells you don't care about to roll a single d6 on a spell you do.
And/or maybe successful spell research 'aimed' at a spell you already possess raises the comprehension by one level.
ISSUE #5: NPCs
Honestly, I would probably ignore this system or streamline it considerably for NPCs. I prefer giving enemy wizards Weird Powers that don't fit the magic system designed to make sense to the players.
ISSUE #6: Level 6 Mastery will fuck up your campaign.
Your campaign world probably could use a good fucking up.
"Man, is there anything Jeff CAN'T do when it comes to gaming? This guy is like a critical 20 every roll. Jeff can bite the heads offa five game geeks, including their sorry-ass DM, and spit 'em into a large duffel bag ONE AT A TIME!...that's just the kind of messed up bastard he is! You think yer a gamer, punk? Well..do ya? Jeff will depants your weasel-ass right in front of your grandma."