I recommend reading chronologically
Chronological order is usually the best way to avoid spoilers (except in books with lots of time travel, or where characters can see the future…which come to think of it does apply here). This isn’t strictly chronological, though: for some books, it doesn’t make much difference if you read them slightly out of order.
First, read:
The Diary of Luke Castellan
This describes how Luke and Thalia found Annabeth, and as such is set some years before the main in Percy Jackson and the Olympians
Then, start Percy Jackson and the Olympians:
After this, it should be safe to read:
The Two Headed Guidance Counselor
The Library of Deadly Weapons
My Demon Satyr Tea Party
My Personal Zombie Apocalypse
These are interactive novels. I’m not sure where these books are located chronologically, but I don’t think they’ll spoil anything that you wouldn’t have learned in the first book.
We have a short story that occurs after The Lightning Thief:
- Percy Jackson and the Stolen Chariot
Then return to the main series:
Another short story:
Percy Jackson and the Bronze Dragon
This provides useful background for events that occur in The Lost Hero, but if you like surprises you might want to hold off until after that novel.
Back to Percy Jackson and the Olympians
The Titan’s Curse
The Titan’s Curse Graphic Novel
The Battle of the Labyrinth
The Battle of the Labyrinth Graphic Novel
Next is:
Apparently this short story happens during The Red Pyramid:
- Percy Jackson and the Sword of Hades
Then is the second book in the Kane Chronicles:
The Throne of Fire
The Throne of Fire Graphic Novel
The Kane Chronicles Survival Guide
This last book was published before The Serpent’s Shadow, and should therefore be safe to read here
Then:
Then read
Next is probably:
Back to the Kane Chronicles:
After that, we start on Heroes of Olympus:
You might as well read these afterwards:
Percy Jackson’s Greek Gods
Percy Jackson’s Greek Heroes
Demigods and Monsters
Percy Jackson and the Olympians: The Ultimate Guide
Most of these could be read earlier, but are probably best enjoyed once one has gained a full appreciation of the Greek side of the Riordanverse.
Then come:
The Son of Sobek
The Staff of Serapis
The Crown of Ptolemy
Then comes the first entry in Magnus Chase and the Gods of Asgard:
The Sword of Summer
This is out of the order that the chronology in the notes presents it, but The Sword of Summer is definitely set before or concurrently with The Hidden Oracle. Percy references Annabeth being in Boston:
“Annabeth.” I tried to place the name. “She’s the blond scary one?”
“That’s her. I promised her specifically that I wouldn’t get myself
killed while she’s gone.”
“Gone?”
Percy waved vaguely toward the north. “She’s in Boston for a few
weeks. Some family emergency. The point is—”
The Hidden Oracle
Lest we think that this refers to some meeting between Annabeth and Magnus after the events of The Hidden Oracle, The Hammer of Thor references the events of that series:
‘There’s a crisis happening,’ Annabeth said. ‘A god fell to earth as a
human. These evil Roman emperors are back, causing trouble.’
The Hammer of Thor
Then go on to:
Finally:
The Hammer of Thor
The Dark Prophecy
These last one has not been released as of the time this answer was written, and as such must be read last. Also, practically speaking, since it is unreleased I don’t really know where it fits in chronologically.
Note: Graphic novels tend to be less detailed than prose novels, so reading the print version of each novel before the graphic version might help you understand the graphic novel better.
Note : I received a great deal of help from this timeline. It’s not certain, but I’m not going to do all that work over myself.