Aside from the Players Handbook and Dungeon Masters Guide (put down that Unearthed Arcana!), look for some of the following Dragon articles for interesting insights and alternate perspectives on the Magic User:
“Everything You Never Knew About Spell Books” by Gary Gygax. Dragon #62, June 1982, p. 14.
Introduces the concept of cantrips; discusses the standard spell book vs. the travelling spell book; cost and creation of spell books; and what happens if a magic user casts a spell directly from a book.
“The Laws of Magic” by Charles Olsen. Dragon #106, February 1986, p. 8.
Describes the source of magic (the fifth element, the akasa); the relationship between magic-user, cleric, and psionic abilities, and the principles of magic that determine when verbal, somatic, or material components are required. Essential reading for anyone interested in designing their own spells.
“Role-Playing: Realism vs. Game Logic; Spell Points, Vanity Press and Rip-offs” by Gary Gygax. Dragon #12, July 1978, p. 15.
This article isn’t essential to understanding the AD&D magic-user, especially as it was written before the release of AD&D. Nonetheless, it’s a supremely entertaining rant by a young Gary Gygax, filled with invectives against the “asinine” logic of would-be game designers, and the “sophomoric” and “obnoxious” vanity press publishers who were the bloggers of the 1970s. Gygax makes the argument for game logic over “realism,” for the obvious reason that magic can never be “realistic.” His prescient remarks against the idea of spellcasters carrying swords and at-will spellcasting are particularly ironic in light of WotC’s later attempt to introduce those elements in the name of “balance.”
Notably, Gary Gygax also makes explicit the fact that the magic system in Chainmail and D&D was inspired by Jack Vance, and talks about ways to limit the power of magic-users through longer spellcasting times and slower level advancement.
“Spells Between the Covers: Details for delving into magical research” by Bruce Heard. Dragon #82, February 1984, p. 55.
This is the definitive work on magical research in AD&D. The article takes the DMG as its jumping-off point, and goes on to describe exactly how a magic user acquires and maintains the library needed for their research. He makes the distinction between research needed to invent a new spell and research needed to learn a spell which the magic user was not able to copy from a scroll or learn from a mage of higher level.
Probably the most entertaining and useful (to my mind, at least) part of the article is a complete list of all the known magical tomes which can be found in order to research spells listed in the Players Handbook, and a table on which to roll the likelihood that any particular book will be found for sale.
A sampling of the table:
“?Manual Powers Beyond the Life?” by Bigby
(burning hands, Bigby?s interposing hand, Bigby?s forceful hand, Bigby?s grasping hand, Bigby?s crushing hand, Bigby?s clenched fist)
“?Theory of the Invisible Forces”? by Tenser
(push, unseen servant, strength, Tenser?s floating disc)
“?Epic Saga of the Great Conjurers?” by Mordenkainen
(Mordenkainen?s faithful hound, conjure animals, monster summoning VI, cacodemon)
And so on.
In addition, it’s great fun to read the works ofGary Gygax’s favorite author, Jack Vance, whose books inspired the entire magic system; and another book cited by Gygax, one of my favorite fantasy books of all time, “The Face in the Frost” by John Bellairs.