These are my house-rules for Magic in the Weird Opera campaign. Most of the rules are cobbled together from various sources, mostly Swords & Wizardry and Labyrinth Lord.
In Weird Opera, there’s no distinction between arcane and divine spells. However, many spells are kept secret by the Guilds or other powerful groups. For example, most spells on the Druid spell list are kept secret by the Druids of the Ancient Mother. Often the only way to learn such a spell is joining the group, or steal it.
Spells and Spellbooks: A spell is an semi-living thing with a disposition and instincts of its own. When a Mage memorizes a spell, it actually leaps off the parchment into the wizard's mind, where it waits impatiently to be released. During that time it’s no longer in the Mage’s spellbook, leaving behind a blank page.
When it’s released, the spell reappears on the page of the spellbook, unless in the meantime something else was written on that page. If a spell is unable to return to its page, it starts wandering and is lost to the Mage.
Spell Casting: The number of memory slots a Mage has is equal to the sum of his INT bonus and his experience level. All memory slots have equal value. The higher the spell level, the more slots that are required to memorize it. One slot equals one spell level. For example, to memorize a 4th level spell a Mage must use 4 slots.
If you don't have enough slots to even memorize the spell, then it's too advanced for you to attempt to memorize it. Otherwise, there’s no restriction to what spell levels a Mage may memorize. A Mage of 1st level with an Intelligence score of 18 (INT bonus +3) may use his 4 memory slots to memorize a single 4th level spell, for example, as long as he has that spell in his spell book or on a scroll.
Knowing Spells: A beginning Mage’s spellbook contains detect magic plus four other first level spells, determined by the DM. Since the Mage received these spells as an apprentice, the DM should take the Mage’s teacher into account when selecting these spells. Beginning spells do not require a roll to see if the Mage can understand them.
New spells are found during play; a wizard must find the higher-level spells in dungeons or musty libraries and copy them into a spellbook, or trade copies with other Mages. If a Mage finds scrolls of spells or other Mages’ spell books while adventuring, these spells can be added to the Mage’s spellbook. There is no limit to how many spells a wizard can learn at a given level.
Check each new spell to see if the Mage can learn and know it, by making an Intelligence check modified by the spell's level. For example, to learn a new 5th level spell, the Mage must make an Intelligence check with -5 penalty. Each time the magician gains a level (if the DM permits) he or she may re-check the spells not understood before, to see if increased experience has granted new understanding.
High-level Magic: In Weird Opera, spells go up only to 6th level. All the higher-level spells are rituals. To learn and cast a ritual spell is a serious undertaking of magic, requiring research, adventuring, and the expenditure of huge quantities of gold. There are books to be found and studied, expensive arcane components to locate, particular times of the year or lunar cycle when the magic can be performed, runes to know, circles to scribe, and other strange and forbidden knowledge to be researched.
As a rule of thumb, learning a ritual should cost at least 1,000 gp, and casting the spell would require about 500 gp per spell level.