I wanted the three Arcane magic classes to be as distinct as possible with little overlap - this would emphasize the differences in play approach of the classes. Illusionists just don't deal out the same kind of damage as a mage would for example, their play style should be more subtle. Druids would be experts at practical applications of magic, especially in the natural world. Mages would have a wider array of magics and probably be more skilled at dealing with raw magic forces. Also Arcane magic has to be real distinct from Divine magic - a decision that led me to include a lot of Druid spells into the Arcane lists, and eventually decide that Druids just weren't going to be Divine spell casters. For the most part I did like the lists used in Microlite (which seem to be a paraphrase of the SRD spell lists), but there was some overlap and sometimes a spell's power would make it useless or overpowered in a point system where you could get away with it in a slots per day system.
The concept I wanted to get across was that Arcane magic was all about complicated and precise thought formulas - the kind of formulas that you would need to have in a book to constantly study and practice. Arcane magic is about the written word. Only Arcane magic can be made into scrolls.
Arcane casters, be they a mage, druid or illusionist, need a spell book and must spend time studying it and working with their craft. Now the spellbook itself isn't magic (although it probably would show faintly if you cast detect magic), however it is written in the language of magic and it's not just a book - it's also a quill and special ink and herbs and notes and lists written on the study of magic. If you lost your spell books and equipment you would barely get by until it was replaced. Just like you could possibly run a game of say BattleTech without the rule books and minis, it is a lot easier with them, especially a large game. Especially a large game in the dark in a foreign language.
Microlite has spell casters knowing all spells of the level they can cast. I like to scale that back quite a bit because I like making spells part of the loot. Beacon rules state that Arcane casters start out knowing their class cantrips and and three 1st level spells. The rest they must seek out in the libraries, dark catacombs and wizarding associations of the world. The great beauty of this kind of thing is that you can totally slide in your own weird 3rd level spell in a old library deep in an ancient crypt.
I don't think I want to write specific rules for gaining new spells or what happens when your spell book is lost because that I think is a setting kind of thing, however personally I think I would be making it pretty hard on characters to cast spells beyond the simple cantrips. In The Sleeping Dragon by Joel Rosenberg, the spell casters must visit the Library in the great city of Pandathaway to transcribe some spells that were lost when their spell books were destroyed. It was a very expensive and time consuming process as befits all things relating to the study of Arcane Magic.
note: I read this cool blogpost about Vancian magic not long after I posted this. Sweet.