DnD Head Canons

[PairO'Dice Lost]

The exact means of preparing a given wizard spell shifts from day to day, from place to place, and from person to person (due to a variety of reasons relating to ley lines, weather, individual physiology, etc.)

The Magicians series by Lev Grossman uses similar fluff for magic:

[Learning magic] turned out to be about as tedious as it was possible for the study of powerful and mysterious supernatural forces to be. The same way a verb has to agree with its subject, it turned out, even the simplest spell had to be modified and tweaked and inflected to agree with the time of day, the phase of the moon, the intention and purpose and precise circumstances of its casting, and a hundred other factors, all of which were tabulated in volumes of tables and charts and diagrams printed in microscopic jewel type on huge yellowing elephant-folio pages. And half of each page was taken up with footnotes listing the exceptions and irregularities and special cases, all of which had to be committed to memory, too. Magic was a lot wonkier than Quentin thought it would be.

"You are here to internalize the essential mechanisms of magic. You think"—[Professor Mayakovsky's] accent made it theenk—"that you have been studying magic....You have practiced your Popper and memorized your conjugations and declensions and modifications. What are the five Tertiary Circumstances?"

It popped out automatically. "Altitude, Age, Position of the Pleiades, Phase of the Moon, Nearest Body of Water."

"Very good," he said sarcastically. "Magnificent. You are a genius."
[...]
"You have been studying magic the way a parrot studies Shakespeare. You recite it like you are saying the Pledge of Allegiance. But you do not understand it...You cannot study magic. You cannot learn it. You must ingest it. Digest it. You must merge with it. And it with you.

"When a magician casts a spell, he does not first mentally review the Major, Minor, Tertiary, and Quaternary Circumstances. He does not search his soul to determine the phase of the moon, and the nearest body of water, and the last time he wiped his ass. When he wishes to cast a spell he simply casts it. When he wishes to fly, he simply flies. When he wants the dishes done, they simply are."

The man muttered something, tapped once resonantly on the table, and the dishes began noisily arranging themselves into stacks as if they were magnetized.

"You need to do more than memorize, Quentin. You must learn the principles of magic with more than your head. You must learn them with your bones, with your blood, your liver, your heart."

It's not my preferred explanation for how spell preparation work, but it's definitely good for in-character magibabble.

A wizard's spellbook is a minor magic item which constantly updates itself to reflect what its owner needs to do to prepare a spell at the current time and place. [...] This is the reason why[...]a wizard who prepares the same spell every day still can't prepare it from memory.

Since D&D magic is, canonically, heavily centered around spoken and written magical language (power words, true names, Words of Creation, spell scrolls, runes, sigils, glyphs...), the flavor I usually use to explain the use of spellbooks is that spell preparation involves the parts of the brain related to reading and language much more than the parts relating to visualization, willpower, or whatever. Thus, even if a wizard can rattle off all the details of a spell from memory, the physical act of reading a spell's writeup in a spellbook makes it much faster and easier to prepare the spell by engaging those parts of the brain automatically and subconsciously. It's much like how memorizing a speech or remembering something someone said or the like takes some effort, but you don't need to think about reading things, you just do it, or how e.g. an expert juggler can try to constantly and consciously think of all six balls he's juggling but it's much easier to rely mostly on muscle memory to keep everything flowing smoothly.

The special inks and such, then, are there to give different visual, tactile, and maybe even olfactory properties to the text for more efficient information storage, to engage slightly different parts of the brain, and to convey metatextual information, much like how if you've been on this forum for a while you "hear" blue text as sarcasm because that totally makes sense, without someone having to add a bunch of qualifiers to get the tone across.

We know the spellbook is at least partially just a mnemonic aid, as it's entirely possible to prepare spells without the spellbook, from the basic read magic spell every wizard can prepare from memory to Spell Mastery that lets a wizard focus on a few spells and internalize them without needing the shortcuts. But it's quite difficult beyond a certain point (the Eidetic Spellcaster ACF requires a wizard to use literal mind-altering substances to form connections between the language and visualization portions of the brain to get the same benefits normally gained by simply reading a spellbook), hence why most wizards just stick with spellbooks.

Also the D&D universe might exist, but it has vastly different rules than our own universe.

Actually, canonically Earth exists in D&D as one of many Material Plane worlds, and the lack of magic and difficulty in accessing other planes is just a quirk of the local physics like any other sealed crystal sphere.

Many of the gods and peoples in the Forgotten Realms originated on Earth, one of the two reasons for the setting's name being that supposedly there were lots of connections between Earth and Toril thousands to hundreds of years ago, and the connections to the foreign realms have faded and been forgotten as various creatures left Earth; various settings (such as d20 Past) and modules (such as The Immortal Storm) take place on, or send people to, alternate Earths; and Mordenkainen of Oerth, Elminster of Toril, Dalamar of Krynn, and Ed Greenwood of Earth used to get together for the occasional chat, as recorded in the Wizards Three articles in Dragon Magazine.