Magic, the Card Game (no relation)

Wizards keep small books of trading cards that are tightly-scribed scrolls. So long as you know the same arcane languages you can trade the cards to build a collection that doubles as your spellbook. Most wizards have a kind of reader with a magnifying glass that allows them to read the cards. More complex spells are rarer, in part because they have to be written smaller. Further, many spells have varying rarity for arbitrary reasons, or due to day-to-day usefulness.