Edit in March 2021 to add the following:
I'm not going to attempt to update this badly dated list. The best reading lists are on my AO3 series pages. The items below are useful, particularly for things that were never added to AO3 (and which I should probably get to at some point).
( Never tell me the word count )On Fanfiction.net
On Archive of our Own
The Other Stuff
Liliandil and the Just King aka it's not all about you
Kwong Lee is a bad Buddhist
Lie Back and Think of Narnia -- Peridan, Flobber and meaning of the word "fark"
The Horse, his boy, the girl, her horse, the Rat and the Trickster stories are here and here
You need a wolf to run a mole to ground -- The Stone Gryphon AU
Morgan and Edmund On Two Hearts Day in the Lone Islands and Acceptance of the terms (after the departure of the Four)
Comment Fic, Give the Pevensies A Friend
More Comment Fic, including why Edmund and Leszi don't get along, why Morgan and Peter don't get along, and Jalur and the Cub.
And Then The World Blew Up (only here because really, how can you resist Sherlock Holmes, L.King's Mary Russell (no relation to Mary Anning Russell of TSG -- a coincidence), Ramses Emerson and Edmund Pevensie in Jerusalem in 1918? The F List and I couldn't and comment fic madness resulted. The only thing missing was Miles Vorkosigan and maybe his mother.)
Lots and lots of Alternate Universe Stone Gryphon stories (where everyone lives and nobody dies and there are adventures) are scattered all over this LJ and spread to others. The easiest way to find out about the camels in the Sahara, spies in Moscow, alligators, and Che Guevara is to go to the TSG AU tag
Eustace, Jill and Quebec
The Great Mother's Day Debate in which the Just King argues for posting Chapter 8 of By Royal Decree so that he can have sex and accuses Rthstewart of disrupting expectations because she's worried her children will grow up to be like him.
Food for Thought, in which I channel the mind and writing of a 16 year old girl who blogs about going through her Great Grandmother Susan's wardrobe.
Because people sometimes ask, On the Narnia side, you have, in chronological order:
The Palace Guard (chronologically first but written after By Royal Decree)
By Royal Decree and at AO3
Chapter 1 of It’s The Thought That Counts
Black as Rat and Crow
Harold and Morgan: Not A Romance (in progress, maybe)
Two Hearts Day (part of H&M: Not A Romance)
Acceptance of the Terms (part of H&M: Not A Romance)
Deny the Child (sequel to Acceptance of the Terms by the awesome Anastigmat)
I love not the man less but nature more the Great Bonding of The Monarchs To Narniacomes first chronologically but I do not recommend reading it until you've been through at least the Narnia stories and probably Part 1 of TSG. And maybe not even then. It's not for everyone, though its assumptions are sprinkled throughout the overall story arc.
On the Spare Oom side you have, in chronological order,
The Stone Gryphon, Part 1, Oxfordshire 1942 and at AO3Chapter 2, It’s The Thought That Counts
The Stone Gryphon, Part 2, The Queen Susan in Tashbaan and at AO3
The Stone Gryphon, Part 3, Apostolic Way
(up to Chapter 15)
Rat and Sword Go To War
I personally think that before you start Part 2 of The Queen Susan in Tashbaan, it's a good idea to skim By Royal Decree and Harold and Morgan (even though unfinished) because there’s a lot that happens there, especially in terms of Edmund’s story, that is relevant. The Palace Guard is relevant to the Good Beasts and other personalities who crop up throughout The Stone Gryphon. I don't have many true surprises but one of them is spoiled in Chapter 2 of the It's The Thought that Counts so you might want to read Chapter 14 of The Stone Gryphon, Part 1, Oxfordshire 1942 first.
Stories outside this (intended to be) cohesive vision are:
Under Cover and at AO3The Maenad of the Maquis and at AO3
Follow The Star and at AO3
Oh My Gawd, it's sooo long, so here's a short cut
A Timeline of events in The Queen Susan in Tashbaan
Partial cast of characters through most of The Queen Susan in Tashbaan, including character key to the allegorical code used in the story.