Back when I was playing in Lew Pulsipher's Tonilda campaign I had a magic-user named Jonas. Jonas had good stats and generally good luck with one exception - he kept blowing the Learn Spell rolls for the 'best' spells. He failed to learn Charm Person, Burning Hands, Stinking Cloud, Web, Lightning Bolt, and (cruelest of all) Fireball.
He knew Magic Missile, though.
On one adventure I found a Wand of Magic Missiles and I later traded some items with a cleric for his Ring of Spell Storing that could hold 4 Magic Missile spells. Then I finally got average and learned Minute Meteors! While I might not have any area of effect spells, I could throw a lot of magical damage down range. While it was fun playing the 'gatling mage' I had glaring holes in my spell combat abilities. I tended to be in parties with two magic-users so we could 'cover for' each other's weaknesses.
While roleplaying with the crew one evening I got to thinking about how darn difficult it must be 'in-universe' to be a mid-level magic-user like Jonas (then 5th level); effectively no melee abilities; some arcane power, but rather limited; and you have a lot of things that much more powerful mages might want. Tonilda was rife with vile clerics and evil mages of one stripe or another lurking about (especially the dreaded pyromancers from Traprain Law) so this was a credible threat. Since the campaign had multiple parties in it, we had a fair number of mid-level magic-users, illusionists, and multi-class types, as well. On the spur of the moment I proposed we band together into a mage's guild.
To the surprise of Lew and especially of me, everyone liked the idea. Before I knew it virtually every spell caster in multiple parties was either in or asking to join. We pooled our resources and built a stronghold with a library and lab; hired 2 alchemists; made copies of all of our spell books and stashed them away; set up bylaws and rules; the whole nine yards. Not only was it great role playing and great fun, it clearly illustrated why guild work in the Real World - strength in numbers. Before too long the guildsmen were adventuring as a group with their henchmen and hirelings along as the muscle.
Never one to miss the extremely obvious when prompted, I soon added an NPC Mage Guild to my own Seaward campaign. I had it run by NPCs, of course, and made it an older institution expanding into the core campaign area. I also gave it a few dark secrets and plot hooks and let it go, hoping my players would take the bait. They did, with enthusiasm. The Guild has been an element of my campaign ever since - 22 years, now, acting as everything from a place where you can get a reliable Identify or sell a wand to advice on how to find a lost command word.
Mage Guild is a book that captures the essence of these guilds. It describes ways to introduce the guild, either as a brand new idea or as an older institution moving into the area. It includes the bylaws, hierarchy and structure, benefits and fees, and rules of the guild. It also has write ups of the guildmaster, the members of the guild council, miscellaneous members of the guild, and a few others who may be friends or foes of the guildsmen..
There are also new familiars, new magic items, and a ton of new spells.
We've done our best to make Mage Guild the way I like the supplements I buy - you can use it as-is, change a few things, or just take what you lie.