[I've decided to take out the "with Jeff Rients" part of this series title. I'm not sure if it was leading people here expecting something totally different and whether that was totally fair of me or not. Sorry if that bothered anyone. No one contacted me about this, but it's just been bothering me.]
So, Jeff's ninth question is: "Is there a magic guild my MU belongs to or that I can join in order to get more spells?"
Answer 1: Yes! It's called the Order of the Green Hand. You can have access to any spells that the other members have, as all newly discovered spells are copied into guild-owned spell books so other guild members can copy them. There are also some other serious perks, but be aware that membership is also costly.
Answer 2: No! Magic is a rare thing and wizards are Dying Earth style selfish, scheming connivers who won't give you a lick of help unless there's some benefit for them, and even then will be scheming to figure out some way to get out of helping you. The way they see it, magical ability is a zero-sum game, so if you're learning more, that must be bad for them unless they're learning more as well. If they learn more while you don't, they must be winning. Your best bet for finding new spells is adventuring in unexplored areas. Sorry, buddy, but dungeons are more accommodating than your fellow magic users.
Answer 3: No, but your first level magic user is almost certainly an apprentice to a higher-level magic user. Your magic user can get spells, advice and magical assistance from this master wizard, but only for as long as your magic user is acting like an apprentice. That means sharing any spells your MU finds, taking quests assigned by the master and possibly turning over at least some of the magic items that your MU finds. That's not all bad, especially since quests to find lost magic and treasure are kind of what this game is about, but eventually your MU will want to strike out on his own and make his way by himself, after which help from your master will be infrequent at best, though he might also give your MU a large and useful gift as a parting present. Oh, and I, as the ref, totally reserve the right to kill your MU's master once you're good and emotionally attached to him; it'll make you want to go after whatever NPC killed him.
Answer 4: No, but that's no biggie for your magic user. He was educated at a magic-user school, where the text was the Book of First Level Spells. When he's ready, any magic-user school library will be happy to sell him the Book of Second Level Spells, the Book of Third Level Spells and so on, at a hefty price, of course.
At the referee's option, certain spells may not be in the general corpus of knowledge, having been lost or just never getting into general circulation; these can be found through adventuring in unexplored areas and can give a magic user great power, as they can be the only ones who can cast the spell. They may contribute the spell to the magic user community, gaining prestige, undying thanks, honorary doctorates, favors and perks, or they may keep it for themselves, gaining a reputation as a magic user who practices spells inscrutable to even other magic users.
As another option, different schools of magic could be rivals with each other, each having different spells in their Book of First Level Spells, which they guard jealously from each other. In order to buy a book of spells from a school's book store, one must have graduated from that school. Selling copies of your spell books to other schools would be one of the highest acts of betrayal to your school, while stealing a copy of another school's spellbooks and turning it over to your school would be a coup that would earn you all sorts of accolades.
Answer 5: No. Why would you need a guild when you can go to the library to look this stuff up? Spells are not exactly common knowledge, but they are common enough that access to them is free. You still need expensive ink to copy them, though, and the library books aren't written/printed in magic ink/gold/giant squid ink/whatever; they just have the information you need to copy it down, so it's no use stealing them.
These five answers aren't all mutually exclusive, and could be combine with each other in various ways. For example, your setting could combine Answers 1, 2 and 3 by having the default Machiavellian magic using culture of Answer 2, but have an upstart Order of the Green Hand (Answer 1) that, firstly, greatly benefits the high-level magic users who run the guild and, secondly, is introducing a new spirit of cooperation into the magic-user culture. If an MU PC's player decides not to join the Order, the MU can be an apprentice to another magic-user (Answer 3), but the master-apprentice relationship is one fraught with both sides seeking advantage over the other and trying to help the other just enough so that the other side doesn't terminate the relationship.
Another possibility could be mixing Answers 4 and 5. Roll randomly on the first three spell levels ten times each. These spells are public knowledge and can be found in the library. For each school of magic, roll a few more times (depending on how big your spell lists are) on each spell level list and those spells are the ones that each particular school has and teaches in addition to the common knowledge spells. Once you get above third level spells, they aren't public knowledge anymore, so if your MU didn't go to school and just studied magic on his own in the library, or was an apprentice, then he's going to have to figure out some way to get his hands on fourth level and higher spells…
What's also interesting to me is how these answers, or any combination of them, immediately sets the tone for how much magic saturates your setting. Low-magic or low-magic-saturation settings have answers like 2 and 3, while high-magic-saturation settings have answers like Answer 5.
So, how do you handle this in your own campaign? What other possibile answers are there to this question? Can you think of a magic user guild that has less costs and benefits than the Order of the Green Hand, but still isn't just handing the MU PC some magical advantages without making them pay for them? Or should MUs really just be given a break and not be asked to pay much for the benefits of guild membership?