A Magic-user can cast spells in two basic ways:
- From a scroll or spellbook, which results in the destruction of the scroll/spellbook entry
- From memory
As I recall, there are no set rules for how long it actually takes to memorize spells in OD&D, but we pretty much go with the 15 minutes per spell level formula. So, scrolls notwithstanding, if you find yourself needing to cast a spell you don't have memorized, your choices are to take the time to memorize it (IF the referee allows it based on how long since you last rested) or you can cast it direct from the spellbook, but lose the spell. I have another idea.
The Magic-user can "ritual cast" direct from the spellbook, without losing the spell. It takes 1 minute per spell level (I use the 10-second combat round, adjust that casting time to best suit your combat round). Casting in this way still uses a spell slot of the appropriate level, so if you suddenly feel the need to cast Light but have already used all your 1st level spells, too bad.
Obviously, this is not something to be used in combat when you discover that your Fireballs are useless and what you really need is a Lightning Bolt. It does off the Magic-user a little versatility outside combat, though. In my estimation that helps keep the "mysterious wizard" from simply being artillery.