No spoilers, but I want to tag this to be safe. There's a lot to say.

[Novels]Your best bets are the Sword Demon trilogy (Ex 2 -> Ex 3 -> Sword Demon Battle Ballad), Ex 4, Sword Identity, and Ex 5. You'll be able to quickly notice the importance of Ex 4, Sword Identity, and Ex 5, but the Sword Demon trilogy's relevance isn't instantly noticeable until later on. As for a reading order, you can read Ex 5 at any time, but as Sword Identity is a sequel to Ex 4, you'll only want to read it after Ex 4. I put the reading order for the trilogy above. There's also the Priscilla Camp side stories, which you should probably only read after Ex 2 because of some guy who appears in both. I'll put my personal reading order below this.

[Novels]Ex 5 -> Ex 4 -> Sword Identity -> Ex 2 -> Priscilla Camp side stories -> Ex 3/Sword Demon Battle Ballad

While you must officially purchase the Ex novels in order to read them, every other side story can be found at this page linking to the Subreddit's translation status area.