OK, looking through the main books for Sorcery Schools/Cults, we have a number of schools with vastly different spells.
Some have access to a lot of spells, others to a few spells. Does this seem fair? Well, yes, it does. Not all cults are the same, some are more powerful than others.
Also, don't forget that most of the Orders and Schools also belong to the True Malkioni Church, so have access to their spells as well. Think of them as specialising in certain magics but still belonging to the main church.
Unless otherwise stated, they can learn spells from other places as well.
I don't like the general idea that all sorcerers have access to all the spells in all the books, which is why the RQ Spellbook is definitely iffy in my opinion. I like the idea of certain Schools or Orders having specialist spells and finding it difficult to learn all common spells.
Having said that, many of the common spells will be generally available, but not from religious sources, which is what the Malkioni Churches, Schools and Orders are. You could put forward the argument that Schools are not particularly religious but in the Middle Sea Empire most of them are state-sponsored in one way or another.
I hope that helps and I haven't confused matters even more.
Jrustela:
The Saintly Bookbearers:
3 Rune Spells, 4 sorcery spells, All spells from the Abiding Book
The Order of Saint Serezos:
2 sorcery spells, All spells from the Abiding Book
The Brothers of Charitable Thought:
2 Rune Spells, 7 sorcery spells, All spells from the Abiding Book, all common sorcery spells
The New Iron Staff:
3 Rune Spells, 14 sorcery spells, All spells from the Abiding Book, All Common spells
Monks of Blood:
3 Rune Spells, 12 Sorcery Spells
Cults of Glorantha 2:
Arkat Kingtroll:
4 Rune Spells, 8 Sorcery Spells, 2 Charms
Atroxic Church:
4 Rune Spells, 10 sorcery spells, probably most of the spells in the Abiding Book through the Book of Betrayal and Murder
Bardan's Book:
3 Rune Spells, 13 sorcery spells
Cogs of Zistor:
4 Rune Spells, 5 Sorcery Spells
Cricible of Iron:
5 Rune Spells, 11 Sorcery Spells
Knights of Saint Volanc:
4 Rune Spells, 11 Sorcery Spells
Malkioni True Church:
All spells from the Abiding Book
Mostal (Rock Dwarves):
6 Sorcery Spells
Mostal (Lead Dwarves):
5 Sorcery Spells
Mostal (Quicksilver Dwarves):
4 Sorcery Spells
Mostal (Copper Dwarves):
4 Sorcery Spells
Mostal (Tin Dwarves):
5 Sorcery Spells
Mostal (Brass Dwarves):
5 Sorcery Spells
Mostal (Silver Dwarves):
All common sorcery spells, all common enchantments
Mostal (Gold Dwarves):
7 Sorcery Spells
Mostal (Iron Dwarves):
10 Sorcery Spells
Mostal (Diamond Dwarves):
0 Sorcery Spells
Order of Gark:
4 Rune Spells, 13 Sorcery Spells
Order of Malkinus the Learned:
3 Rune Spells, 20 Sorcery Spells
Saint Waertag:
3 Rune Spells, 11 Sorcery Spells
School of Hwarosian Mysteries:
4 Rune Spells, 9 Sorcery Spells, All Common Sorcery Spells, All Spells from the Abiding Book
Xemelite Nuns:
4 Rune Spells, 10 Sorcery Spells, Access to all Healing Rune/Sorcery Magic from all other Malkioni Schools except Atroxic and Malkinius