detrosabiaz:

Insomnia means that I have spent two hours on the Choices wiki trying to figure out how all of these books connect to each other and it has led me to conclude:

• The Crown & The Flame is a television show based off of a book series. 
  
• It is my firmly held belief that the book series is a mix of real life events and legend - or at least, folks are led to believe that the fire powers are legend, since normal humans probably couldn’t handle a Fire-Att running around. 
  
• It is also a video game played by the characters of High School Story.
• The Five Kingdoms (C&F) united together to form Cordonia (Rules of Engagement, The Royal Romance).
  
• Leo and Liam of Rules of Engagement and The Royal Romance, respectively, are descendants of Kenna Rhys (C&F), while Olivia Nevrakis (TRR) is a direct descendant of Zenobia Nevrakis (C&F). 
  
• Leo steps down as Crown Prince of Cordonia in Rules of Engagement, leaving his brother Liam to have to choose a wife and take the throne in The Royal Romance.
• Update courtesy of an awesome anon: Madeleine’s father from TRR is the Duke of Karlington, which was the same title Duke Richards has in Desire & Decorum.
• The main character from Rules of Engagement meets and interacts with characters from Most Wanted and The Freshman.
  
• The cruiseline from ROE is used in High School Story.
• Characters in HSS appear to engage in an MMO against a group of characters from The Freshman.
  
• Hartfield University, the college at which The Freshman, Sophomore, Junior, and Senior all take place, is the alma mater of characters in #LoveHacks and Veil of Secrets.
  
• Hartfield is also where the main characters of The Haunting of Briarwood Manor and Endless Summer attend, and is the school that the main character in The Elementalists drops out from before going to Penderghast.
  
• The scarab from the beginning of Bloodbound, or one very similar, is sought after by Griffin in the beginning of The Elementalists.
• At Penderghast (TE), the library has textbooks citing the Kraken (C&F), Dopey Cat (#LH), and Redfield (It Lives in the Woods). A Baby Kraken can also be found in Endless Summer.
  
• Second update from the awesome anon: Hero’s Silas Prescott has a crystal said to be from Endless Summer’s La Huerta, said to be shaped like the island’s heart.
• In The Elementalists, everyone has an Attunement, allowing you to have a connection and some control over a certain element, like fire (C&F).
  
• ClickIt, the website for which the main character of #LoveHacks works, is used by characters in HSS and is mentioned at the beginning of Big Sky Country.
  
• Erin in High School Story appears to be enamored with Victoria Fontaine, a character in Red Carpet Diaries. 
  
• The characters in HSS are also aware of events outside their country, as Emma references Liam not choosing the waitress at the coronation ball (TRR). 
• Chris Wood and Thomas Hunt (RCD) are guest judges for a challenge in America’s Most Eligible. 
  
• Becca Davenport (The Senior) mentions watching America’s Most Eligible, and the main character of Perfect Match believes the show would be more enjoyable if the eliminated contestants were dropped down a trap door. 
  
• The contestants of America’s Most Eligible say that they enjoy watching The Crown & The Flame, as do characters in Endless Summer.
  
• Characters in The Freshman are avid fans of the C&F series, and even visit the set in LA, where they’re filming the final season. Here they meet characters from Most Wanted.
  
• A song written by Hayley Rose during the events of Most Wanted is heard in The Freshman. 
• Alyssa Griffin is an actress from Most Wanted who transitions to singing by the time of The Freshman, which puts the events of Most Wanted before The Freshman. Certain spoilers from Endless Summer set it after the events of most other books in this universe. Alyssa also makes appearances in ROE and PM.
  
• Perfect Match has its characters visit the Double Tap, a bar frequented by the characters of #LH, and Damien mentions he grew up reading the comics for which Ben Parks (#LH) is an illustrator. 
  
• Most Wanted 2 involves a series of murders perpetrated by a cult, which inspires some kind of adaptation (book or tv show, I’m assuming) that inspires an arcade game in Endless Summer.
• Characters from Most Wanted appear in Red Carpet Diaries, as does a blog that sets Most Wanted prior to the events of RCD.
  

I am tired, so I probably missed more ties between all of these books. I know I didn’t manage to fit all of the books/series in here (Looking at you, A Courtesan of Rome), but this is what I’ve got.