From my readings of Vance, it seems that magicians don't really get magic. They can learn spells by rote, and mostly they learn their spells from finding, capturing, or trading with other magic-users. Think about it: in the D&D Cyclopedia, there are 117 magic-user spells. A magic-users learns, what, one or two when he advances a level? If a magic-user started with three and advanced to 20th level, gaining two spells at every level, he would know only 43 spells. Even if he advanced to 36th level, he would know only 75 of the 117 commonly known spells. Most magic-users don't make it to 36th level. My point is that a magic-user who does nothing but his own research and reaches the pinnacle of human ability will still know less than 75% of all magical spells. A wizard must beg, borrow, steal, and cajole if he ever hopes to learn them all, much like Mazirian the Magician planned to do.
Where I am going with this, and it is stated as much in Vance, (can't remember the story or actual quotes) that there are only 100 spells or so known to mankind, out of the thousands that once apparently existed. In addition, magicians of the Dying Earth only know their rote-spells and are generally incapable of devising their own. (Unless you're one of the great ones like Phandal, but he lives in another universe for crying out loud.) In the rules of D&D as written, a magic-user has to use a spell to decipher a discovered scroll or spell book; even spells that he has in his own spell book are not recognizable when first encountered. (I have changed this in my AD&D game, as posted previously under house rules.)
Magic-users are simply that; users of magic. They do not have a deep understanding of the principles behind magic. They scavenge and steal spells because their limited ability to learn means they must devote a lifetime of research if they hope to even understand most spells, and they will never know a spell well enough that their mind can hold onto it after the energies are discharged, nor will it ever be familiar enough that they can spot it in another wizard's spell book.
Now, in my campaign, magic-users can decipher spells without resorting to read magic, so long as the spell is within their ability to cast. Read Magic can, however, decipher spells which are beyond the magic-user's ability. Magic-users do understand some of the basic principles of magical runes, but a lot of magical lore/science has been lost over the ages. For the most part, magic-users still do things by rote, but in my campaign, magic runes and writings are consistent enough that you can figure out a spell you know, even if the other wizard arrives at the same spell through different applications of magical runes and formula. Magic is simply not something for which the mortal intellect is suited, and even the wisest magic-user is a babe in arms when faced with the totality of magical lore.
...so where did all that lore come from, and what led to the loss of it?
We shall see, both in further readings (perhaps) and in my campaign.