I feel I am getting closer to specifically defining how a spell works in a character's mind as well as in within the structure of my rules. The description below corresponds to all my classes - it is only that the content below has been taken from my recently reworked illusionist page.
Unlike other systems, illusionists in my world cannot change or alter their spells daily. The list of spells they possess for their characters are the only spells those characters understand. Thus they must choose carefully at each level, then make the best use of the spells they possess as they can. Like a fighter able to use only a specific list of weapons & skills, the illusionist (and any other spellcaster) is only able to use a specific list of spells.
After casting a spell (or cantrip, for it is true in the case of both), the illusionist must spend a period of time in concentration and study before the spell can be cast again. This is often referred to as 'memorization,' but more accurately it is an ordering of the mind. When the spell is cast, the mind is disordered and much energy is expended. This is sometimes wrongly called 'forgetting' the spell. In fact it is simply that the discipline necessary to recast has been lost. Because of so much energy lost, the illusionist must rest for at least six hours before the necessary time can be taken to discipline the mind again, in the precise manner needed. This process is sometimes called 'learning the spell.'
To an outsider, it really does seem as though the illusionist must relearn the spell every day through memorization, before forgetting the spell again. However, these are the simple-minded explanations given to spellcasting by those who are incapable of truly understanding it.
The illusionist need not take steps to reorder the same spell every day; once compartmentalized in the mind, it is not lost until the spell is actually cast. Thus the unused spell remains locked in the illusionist's mind, unless some damage is caused to the brain that would result in the loss of the spell (an injury that would damage the illusionist's memory or some form of madness). Otherwise, the spell remains fixed until the necessary key words are concentrated upon, 'freeing' the spell. This freeing process is called 'casting.' The process of then letting go of the spell (causing it to enact itself) is called 'discharging.'
Casting a spell and unlocking the mass of knowledge about that spell is the process by which energy is gained from the surrounding environment and pulled into the illusionist, providing the necessary power to transform the physical universe in the manner for which the spell has been created.
As a method to enable the concentration and reordering of the spell, the symbols and words are stored on books and scrolls, to which the illusionist must have access. As having a large number of spells and cantrips can take a lot of space, many higher level illusionists acquire several books or scrolls that they keep in various cases - however, as a group, these are jointly called a 'spellbook' - largely because every low-level illusionist begins with just one small book sufficient to keep records for a few cantrips and 1st level spells. As more spells and cantrips are gained, however, inevitably another book is purchased - or if bookbinding is rare in a region and difficult to obtain, scrolls will be used in a book's place.
Spellbooks and scrolls are measured by 'quires,' a measurement that most would associate with 16 book pages - if the book is 9 inches by 12 inches in size. Changing the size of the book page would change the number of spells and cantrips that could be written on a given page . . . so rather than count the space needed to write a spell or cantrip by 'pages,' quires are used.
A single cantrip will require 1/32 of a quire. A given spell level would require 1/16 of a quire. Thus a 3rd level spell would require 3/16 of a quire. As books in the equipment system are priced and measured by the number of quires they contain, the size of a spellbook the player buys at the start of the campaign determines how long it will be before there is no more space and another book or scroll is required. Take note that in gaining several spells and cantrips at a given level, this space can be used up quickly - and it is often true that a low level illusionist can't afford more than a small, cheap book at first level due to available funds.
As space is required, an illusionist's spellbook may include a group of books which are tied together with string or stored in a box—or even several boxes. A very large spell book can have as many as 14 quires, large enough for a 26th level illusionist. Such a book would be very heavy and expensive.
Copying spells from one book to another is entirely possible if the illusionist knows the spell, but the process will become quite expensive in terms of magical ink. For game purposes, the cost of inscribing a new spell gained from levels - the first time - into the illusionist's spellbook is suspended (simply because I do not wish to punish players for going up a level!). I generally presume that the illusionist has been carefully obtaining just enough magical ink to inscribe their book (through the donations of persons well-met or guilds) without this acquisition being noted on their character sheets. Elective spell copying, however, will require the cost of materials to achieve.
It is presumed that while the character is at a certain level and able to cast those spells they understand, they are reflecting upon and considering other spells that they partly understand (due to their original training and insight) until such time as they achieve a level and the necessary epiphany needed to finally understand how those spells work has been acquired. This epiphany is the result of having gained enough experience.
If a spellbook were destroyed, it would be very difficult and expensive for an illusionist to re-gather their spells together again. While the illusionist could simply write out any spells that were still ordered in mind (a process of unlocking the spell without discharging), finding copies of spells that were not ordered (but which the illusionist would understand upon seeing) could require much searching and expense - plus the cost of materials.