Single use spells are stored in small, often consumable forms. Sleep may be stored in a small amount of sand, which must be thrown at the eyes of your foes. Healing spells are stored as liquids to be drank as potions, or could be made as bandages or ointments to keep them separate. Plenty of language based spells or more esoteric spells could be stored as scrolls, which fits the aesthetic of Wizards carrying scrolls around. Spells with multiple uses or that have a duration are just useful items; your spell that lets you see the invisible is enchanted into your monocle.
Spells that are single use but are common but are not as disposed of could be infused in other items instead, called Foci (plural for “Magic Focus”) or Fetish. Feather Fall could be stored in an actual feather which is consumed on the casting, or in a magic-users billowing cape or dress. Offensive spells are stored in wands and staves as normal- with different command words, gestures, or mental acuity to control which spell comes out of the wand. Some Wizards may carry around multiple different wands for different situations, where as others will decorate and closely guard their prized dueling staff; engraved with several runes and having several powerful offensive and defensive spells loaded within at any given time. When a dumb goblin steals the Wizard's wand and bops himself on the head to turn invisible or immortal or whatever he thinks will happen, there is a good chance he might accidentally offload the Fireball instead, killing himself and anyone nearby. Magic Fetishes can also have mutational or wild magic effects when broken or used incorrectly; your magic locket with your illusion and enchantment spells has the perfect opportunity to create some false beings with insane personalities and powers over perception, adding to the feeling of the world as a chaotic and fantastical place.
Spell research works the exact same; except now, spells and research costs are the same. If your game doesn't use costs for components (or doesn't use components to begin with), then instead the research cost is the same as the component cost, since you're securing the magical items you need for your spells. Your magic user has a little farm of the bright green lizards you need to mummify and pickle to cast Regeneration, not just a room filled with spell books.
Spells books are another strange part however; if all magic spells are just items, then what purpose do they serve? Personally, I like “spells per adventure” instead of spells per day, so reloading spells isn't necessary with a spellbook. Instead, spellbooks could still be for preparing spells in items, but be a bit more esoteric and more about the preparation of materials and containing the knowledge of how magic works... like an actual spellbook.
What about magic item longevity? The “shelf life”? This is one issue I have with the concept, essentially saying Wizards could stockpile spells if they had infinite or very long shelf life. If the shelf life is short this could work, but would also diminish the immersion factor somewhat if magic items found in the world only are the permanently enchanted item kind, not the single use spell kind. This could also be abstracted somewhat; some spells degrade faster then others, like lower level spells, and “leaky” spells could radiate magical energy that causes mutations or makes interior spaces bigger on the inside then the outside; which is exactly where the dungeons getting so big and complex comes from. Dungeons as mythical underworld is still a great concept, and this could help explain them in a more naturalistic sense. To keep Wizards from mass producing spells, just abstract your regular spell slot/spells per day limit as the number of magical artifacts and fetishes a Wizard can have and maintain at any one time.
Finally, this comes down to mechanics in the form of inventory. How do you put this in mechanically? Having magic spells being items could be an interesting concept for resource management; you start the adventure loaded with spells and barely able to carry treasure, but as you start to use up your consumable spells you may open up enough space. It might also be fun to see a MU drop a charged up situational or useless spell on the floor of the dungeon so they can help carry a treasure chest; thus ensuring shenanigans when the orcs find it and use it as a weapon against the party. You could also use inventory space as a game balance mechanic; Sleep may be a good spell but may require you to carry around a heavy ass bag of sand instead of the much more useful Magic Missile in your wand. Minor enough to not be a hassle but significant enough to be a choice for certain adventures and campaigns.
Anyway, this article is a bit more spontaneous and less thought out then my others, and is more of a rant. Still this concept is near and dear to my heart and I'd love to hear your thoughts on it, or perhaps inspire others to do something similar with their games. Magic Users are the most fun class to tinker with for good or for ill, so ideas abound.